diff --git a/.github/workflows/unit-tests.yml b/.github/workflows/unit-tests.yml
index 773306e..e93458d 100644
--- a/.github/workflows/unit-tests.yml
+++ b/.github/workflows/unit-tests.yml
@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: ["3.1", "3.2", "3.3", "3.4"]
+        ruby: ["3.1", "3.2", "3.3", "3.4", "4.0"]
         gemfile: [
           dev/gemfiles/rails-7.2.x.gemfile,
           dev/gemfiles/rails-8.0.x.gemfile,
diff --git a/CHANGELOG.md b/CHANGELOG.md
index e4b38f3..0f8d1f1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased]
 
+### Changed
+
+* Add Ruby 4.0 to the test matrix (#23)
+
 ## Version 1.1.0 (2025-10-23)
 
 ### Changed
diff --git a/Gemfile.lock b/Gemfile.lock
index 009592b..275bc42 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -7,9 +7,9 @@ PATH
 GEM
   remote: https://rubygems.org/
   specs:
-    activemodel (8.1.0)
-      activesupport (= 8.1.0)
-    activesupport (8.1.0)
+    activemodel (8.1.1)
+      activesupport (= 8.1.1)
+    activesupport (8.1.1)
       base64
       bigdecimal
       concurrent-ruby (~> 1.0, >= 1.3.1)
@@ -24,28 +24,29 @@ GEM
       uri (>= 0.13.1)
     ast (2.4.3)
     base64 (0.3.0)
-    bigdecimal (3.3.1)
+    bigdecimal (4.0.1)
     camertron-eprun (1.1.1)
     cldr-plurals-runtime-rb (1.1.0)
-    concurrent-ruby (1.3.5)
-    connection_pool (2.5.4)
+    concurrent-ruby (1.3.6)
+    connection_pool (3.0.2)
     diff-lcs (1.6.2)
     drb (2.2.3)
-    i18n (1.14.7)
+    i18n (1.14.8)
       concurrent-ruby (~> 1.0)
-    json (2.15.1)
+    json (2.18.0)
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
     logger (1.7.0)
-    minitest (5.26.0)
+    minitest (6.0.1)
+      prism (~> 1.5)
     parallel (1.27.0)
-    parser (3.3.9.0)
+    parser (3.3.10.0)
       ast (~> 2.4.1)
       racc
-    prism (1.6.0)
+    prism (1.7.0)
     racc (1.8.1)
     rainbow (3.1.1)
-    rake (13.3.0)
+    rake (13.3.1)
     regexp_parser (2.11.3)
     rspec (3.13.2)
       rspec-core (~> 3.13.0)
@@ -56,11 +57,11 @@ GEM
     rspec-expectations (3.13.5)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.13.0)
-    rspec-mocks (3.13.6)
+    rspec-mocks (3.13.7)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.13.0)
     rspec-support (3.13.6)
-    rubocop (1.81.6)
+    rubocop (1.82.1)
       json (~> 2.3)
       language_server-protocol (~> 3.17.0.2)
       lint_roller (~> 1.1.0)
@@ -68,12 +69,12 @@ GEM
       parser (>= 3.3.0.2)
       rainbow (>= 2.2.2, < 4.0)
       regexp_parser (>= 2.9.3, < 3.0)
-      rubocop-ast (>= 1.47.1, < 2.0)
+      rubocop-ast (>= 1.48.0, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 2.4.0, < 4.0)
-    rubocop-ast (1.47.1)
+    rubocop-ast (1.49.0)
       parser (>= 3.3.7.2)
-      prism (~> 1.4)
+      prism (~> 1.7)
     rubocop-performance (1.26.1)
       lint_roller (~> 1.1)
       rubocop (>= 1.75.0, < 2.0)
@@ -81,9 +82,9 @@ GEM
     rubocop-rake (0.7.1)
       lint_roller (~> 1.1)
       rubocop (>= 1.72.1)
-    rubocop-rspec (3.7.0)
+    rubocop-rspec (3.8.0)
       lint_roller (~> 1.1)
-      rubocop (~> 1.72, >= 1.72.1)
+      rubocop (~> 1.81)
     ruby-progressbar (1.13.0)
     securerandom (0.4.1)
     twitter_cldr (6.14.0)
@@ -95,8 +96,8 @@ GEM
       concurrent-ruby (~> 1.0)
     unicode-display_width (3.2.0)
       unicode-emoji (~> 4.1)
-    unicode-emoji (4.1.0)
-    uri (1.0.4)
+    unicode-emoji (4.2.0)
+    uri (1.1.1)
 
 PLATFORMS
   ruby
@@ -104,7 +105,7 @@ PLATFORMS
 DEPENDENCIES
   activemodel (> 3.2.0)
   activesupport (> 3.2.0)
-  bundler (~> 2.1)
+  bundler (>= 2.1)
   postcode_validator!
   rake (~> 13.0)
   rspec (~> 3.13.0)
@@ -114,48 +115,48 @@ DEPENDENCIES
   rubocop-rspec
 
 CHECKSUMS
-  activemodel (8.1.0) sha256=43aa66565a601483e6c614d02cb2090168c91a803ddb954e2b0ef24d22de745e
-  activesupport (8.1.0) sha256=d4adf40725be7bbc2a16936a6cf41b175a46ba0c5ccdb703ea4ed7c8801e070a
+  activemodel (8.1.1) sha256=8b7e2496b9e333ced06248c16a43217b950192c98e0fe3aa117eee21501c6fbd
+  activesupport (8.1.1) sha256=5e92534e8d0c8b8b5e6b16789c69dbea65c1d7b752269f71a39422e9546cea67
   ast (2.4.3) sha256=954615157c1d6a382bc27d690d973195e79db7f55e9765ac7c481c60bdb4d383
   base64 (0.3.0) sha256=27337aeabad6ffae05c265c450490628ef3ebd4b67be58257393227588f5a97b
-  bigdecimal (3.3.1) sha256=eaa01e228be54c4f9f53bf3cc34fe3d5e845c31963e7fcc5bedb05a4e7d52218
+  bigdecimal (4.0.1) sha256=8b07d3d065a9f921c80ceaea7c9d4ae596697295b584c296fe599dd0ad01c4a7
   camertron-eprun (1.1.1) sha256=9a8e337916445e62648f6a4f35d229302fc44c50549cc881bff1e296ccf9ce4b
   cldr-plurals-runtime-rb (1.1.0) sha256=2353875af89be9fac049c60891e973825b24a979eeef7274093c9ff5fd56dfa9
-  concurrent-ruby (1.3.5) sha256=813b3e37aca6df2a21a3b9f1d497f8cbab24a2b94cab325bffe65ee0f6cbebc6
-  connection_pool (2.5.4) sha256=e9e1922327416091f3f6542f5f4446c2a20745276b9aa796dd0bb2fd0ea1e70a
+  concurrent-ruby (1.3.6) sha256=6b56837e1e7e5292f9864f34b69c5a2cbc75c0cf5338f1ce9903d10fa762d5ab
+  connection_pool (3.0.2) sha256=33fff5ba71a12d2aa26cb72b1db8bba2a1a01823559fb01d29eb74c286e62e0a
   diff-lcs (1.6.2) sha256=9ae0d2cba7d4df3075fe8cd8602a8604993efc0dfa934cff568969efb1909962
   drb (2.2.3) sha256=0b00d6fdb50995fe4a45dea13663493c841112e4068656854646f418fda13373
-  i18n (1.14.7) sha256=ceba573f8138ff2c0915427f1fc5bdf4aa3ab8ae88c8ce255eb3ecf0a11a5d0f
-  json (2.15.1) sha256=b1c1b2e7c116eb1903e0ce0c374783e6ead8747a0f9eca132d274018ebb80b89
+  i18n (1.14.8) sha256=285778639134865c5e0f6269e0b818256017e8cde89993fdfcbfb64d088824a5
+  json (2.18.0) sha256=b10506aee4183f5cf49e0efc48073d7b75843ce3782c68dbeb763351c08fd505
   language_server-protocol (3.17.0.5) sha256=fd1e39a51a28bf3eec959379985a72e296e9f9acfce46f6a79d31ca8760803cc
   lint_roller (1.1.0) sha256=2c0c845b632a7d172cb849cc90c1bce937a28c5c8ccccb50dfd46a485003cc87
   logger (1.7.0) sha256=196edec7cc44b66cfb40f9755ce11b392f21f7967696af15d274dde7edff0203
-  minitest (5.26.0) sha256=f5817ad09f863a4f7eac917707c1ca5c09cdc4a35e17d91171760178324d2c30
+  minitest (6.0.1) sha256=7854c74f48e2e975969062833adc4013f249a4b212f5e7b9d5c040bf838d54bb
   parallel (1.27.0) sha256=4ac151e1806b755fb4e2dc2332cbf0e54f2e24ba821ff2d3dcf86bf6dc4ae130
-  parser (3.3.9.0) sha256=94d6929354b1a6e3e1f89d79d4d302cc8f5aa814431a6c9c7e0623335d7687f2
+  parser (3.3.10.0) sha256=ce3587fa5cc55a88c4ba5b2b37621b3329aadf5728f9eafa36bbd121462aabd6
   postcode_validator (1.1.0)
-  prism (1.6.0) sha256=bfc0281a81718c4872346bc858dc84abd3a60cae78336c65ad35c8fbff641c6b
+  prism (1.7.0) sha256=10062f734bf7985c8424c44fac382ac04a58124ea3d220ec3ba9fe4f2da65103
   racc (1.8.1) sha256=4a7f6929691dbec8b5209a0b373bc2614882b55fc5d2e447a21aaa691303d62f
   rainbow (3.1.1) sha256=039491aa3a89f42efa1d6dec2fc4e62ede96eb6acd95e52f1ad581182b79bc6a
-  rake (13.3.0) sha256=96f5092d786ff412c62fde76f793cc0541bd84d2eb579caa529aa8a059934493
+  rake (13.3.1) sha256=8c9e89d09f66a26a01264e7e3480ec0607f0c497a861ef16063604b1b08eb19c
   regexp_parser (2.11.3) sha256=ca13f381a173b7a93450e53459075c9b76a10433caadcb2f1180f2c741fc55a4
   rspec (3.13.2) sha256=206284a08ad798e61f86d7ca3e376718d52c0bc944626b2349266f239f820587
   rspec-core (3.13.6) sha256=a8823c6411667b60a8bca135364351dda34cd55e44ff94c4be4633b37d828b2d
   rspec-expectations (3.13.5) sha256=33a4d3a1d95060aea4c94e9f237030a8f9eae5615e9bd85718fe3a09e4b58836
-  rspec-mocks (3.13.6) sha256=78b137ae5e91c5072bf81d40d78189aa911e965a81adc245e2d8d9ba624d9b4e
+  rspec-mocks (3.13.7) sha256=0979034e64b1d7a838aaaddf12bf065ea4dc40ef3d4c39f01f93ae2c66c62b1c
   rspec-support (3.13.6) sha256=2e8de3702427eab064c9352fe74488cc12a1bfae887ad8b91cba480ec9f8afb2
-  rubocop (1.81.6) sha256=a7095eca1b79bbbe7d81c74c11ed2c532faa868bd9d31c2f12b7be7fbfbfef42
-  rubocop-ast (1.47.1) sha256=592682017855408b046a8190689490763aecea175238232b1b526826349d01ae
+  rubocop (1.82.1) sha256=09f1a6a654a960eda767aebea33e47603080f8e9c9a3f019bf9b94c9cab5e273
+  rubocop-ast (1.49.0) sha256=49c3676d3123a0923d333e20c6c2dbaaae2d2287b475273fddee0c61da9f71fd
   rubocop-performance (1.26.1) sha256=cd19b936ff196df85829d264b522fd4f98b6c89ad271fa52744a8c11b8f71834
   rubocop-rake (0.7.1) sha256=3797f2b6810c3e9df7376c26d5f44f3475eda59eb1adc38e6f62ecf027cbae4d
-  rubocop-rspec (3.7.0) sha256=b7b214da112034db9c6d00f2d811a354847e870f7b6ed2482b29649c3d42058f
+  rubocop-rspec (3.8.0) sha256=28440dccb3f223a9938ca1f946bd3438275b8c6c156dab909e2cb8bc424cab33
   ruby-progressbar (1.13.0) sha256=80fc9c47a9b640d6834e0dc7b3c94c9df37f08cb072b7761e4a71e22cff29b33
   securerandom (0.4.1) sha256=cc5193d414a4341b6e225f0cb4446aceca8e50d5e1888743fac16987638ea0b1
   twitter_cldr (6.14.0) sha256=5d18c23097008bcda542a260535766fdfc49d982d5eedf2486d19f368aa8c7a4
   tzinfo (2.0.6) sha256=8daf828cc77bcf7d63b0e3bdb6caa47e2272dcfaf4fbfe46f8c3a9df087a829b
   unicode-display_width (3.2.0) sha256=0cdd96b5681a5949cdbc2c55e7b420facae74c4aaf9a9815eee1087cb1853c42
-  unicode-emoji (4.1.0) sha256=4997d2d5df1ed4252f4830a9b6e86f932e2013fbff2182a9ce9ccabda4f325a5
-  uri (1.0.4) sha256=34485d137c079f8753a0ca1d883841a7ba2e5fae556e3c30c2aab0dde616344b
+  unicode-emoji (4.2.0) sha256=519e69150f75652e40bf736106cfbc8f0f73aa3fb6a65afe62fefa7f80b0f80f
+  uri (1.1.1) sha256=379fa58d27ffb1387eaada68c749d1426738bd0f654d812fcc07e7568f5c57c6
 
 BUNDLED WITH
-   2.7.2
+  4.0.3
diff --git a/postcode_validator.gemspec b/postcode_validator.gemspec
index 95cf1d1..b472d0f 100644
--- a/postcode_validator.gemspec
+++ b/postcode_validator.gemspec
@@ -45,7 +45,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'activesupport',  '> 3.2.0'
 
   # Development tools
-  spec.add_development_dependency 'bundler',        '~> 2.1'
+  spec.add_development_dependency 'bundler',        '>= 2.1'
   spec.add_development_dependency 'rake',           '~> 13.0'
   spec.add_development_dependency 'rspec',          '~> 3.13.0'
 end