diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index a913540f..2a23608c 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -31,6 +31,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -61,6 +62,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -91,6 +93,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -121,6 +124,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d2c953a8..a65d21e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 * [CHORE] Updated README with last supported Ruby versions for the older as well as the latest releases of Rubycritic/Ruby (by [@faisal][])
 * [CHANGE] Drop support for Ruby 3.1.x and JRuby 9.4 (by [@faisal][])
 * [CHANGE] Add CI support for JRuby 10.0 (by [@faisal][])
+* [CHORE] Start testing library with Ruby 4.0 (by [@etagwerker][])
 * [CHANGE] Bump cucumber dependency (by [@faisal][])
 * [BUGFIX] Fixed regression in compatibility with Flog 4.9.0 (by [@faisal][])
 * [CHANGE] Bump mocha dependency (by [@faisal][])
diff --git a/lib/rubycritic/analysers/smells/reek.rb b/lib/rubycritic/analysers/smells/reek.rb
index db5cf0cf..cfe8ee5e 100644
--- a/lib/rubycritic/analysers/smells/reek.rb
+++ b/lib/rubycritic/analysers/smells/reek.rb
@@ -10,7 +10,10 @@ class ReekSmells
       include Colorize
 
       def initialize(analysed_modules)
-        @analysed_modules = analysed_modules.reject { Reek.configuration.path_excluded?(_1.pathname) }
+        @analysed_modules = analysed_modules.reject do |mod|
+          path_string = mod.pathname.to_s
+          Reek.configuration.path_excluded?(Pathname.new(path_string))
+        end
       end
 
       def run
diff --git a/lib/rubycritic/generators/html/line.rb b/lib/rubycritic/generators/html/line.rb
index c5e3cd15..51fc5e6a 100644
--- a/lib/rubycritic/generators/html/line.rb
+++ b/lib/rubycritic/generators/html/line.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'cgi'
+require 'cgi/escape'
 require 'rubycritic/generators/html/base'
 
 module RubyCritic
diff --git a/rubycritic.gemspec b/rubycritic.gemspec
index ff4adb93..afadd7a3 100644
--- a/rubycritic.gemspec
+++ b/rubycritic.gemspec
@@ -54,8 +54,7 @@ Gem::Specification.new do |spec|
   end
   spec.add_development_dependency 'cucumber', '~> 10.2.0', '>= 10.1.0'
   spec.add_development_dependency 'diff-lcs', '~> 1.3'
-  spec.add_development_dependency 'fakefs', '~> 3.0.0'
-  spec.add_development_dependency 'irb'
+  spec.add_development_dependency 'fakefs', '~> 3.2.0'
   spec.add_development_dependency 'mdl', '~> 0.15.0', '>= 0.12.0'
   spec.add_development_dependency 'minitest', '~> 6.0.0'
   spec.add_development_dependency 'minitest-around', '~> 0.6.0'
diff --git a/test/fakefs_helper.rb b/test/fakefs_helper.rb
index a63f7abc..29d05131 100644
--- a/test/fakefs_helper.rb
+++ b/test/fakefs_helper.rb
@@ -24,3 +24,16 @@ def home(user = Etc.getlogin)
   end
 end
 FakeFS::Dir.singleton_class.prepend(FakeFSPatch)
+
+# Patch FakeFS::Pathname to include the path method for Ruby 4.0.0 compatibility
+# This is needed because Reek's configuration internally calls Pathname#== which
+# requires the path method to be present
+module FakeFS
+  class Pathname
+    undef :path
+
+    def path
+      to_s
+    end
+  end
+end