diff --git a/.github/workflows/pull_request.yaml b/.github/workflows/pull_request.yaml
index 76d00b6..c51d598 100644
--- a/.github/workflows/pull_request.yaml
+++ b/.github/workflows/pull_request.yaml
@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-24.04
     strategy:
       matrix:
-        ruby-version: ['3.0', '3.1', '3.2', '3.3']
+        ruby-version: ['3.0', '3.1', '3.2', '3.3', '3.4', '4.0']
 
     steps:
     - uses: actions/checkout@v3
diff --git a/Gemfile.lock b/Gemfile.lock
index 1ab6839..c316d44 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -11,6 +11,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     ast (2.4.2)
+    benchmark (0.5.0)
     byebug (11.1.3)
     coderay (1.1.3)
     diff-lcs (1.5.0)
@@ -20,7 +21,7 @@ GEM
       logger
     faraday-net_http (3.4.4)
       net-http (~> 0.5)
-    faraday-retry (2.2.1)
+    faraday-retry (2.4.0)
       faraday (~> 2.0)
     gem-release (2.2.2)
     json (2.21.2)
@@ -32,6 +33,7 @@ GEM
     multi_json (1.17.0)
     net-http (0.9.1)
       uri (>= 0.11.1)
+    ostruct (0.6.3)
     parallel (1.24.0)
     parser (3.3.0.4)
       ast (~> 2.4.1)
@@ -82,9 +84,11 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  benchmark
   bundler
   flagsmith!
   gem-release
+  ostruct
   pry
   pry-byebug
   rake
diff --git a/flagsmith.gemspec b/flagsmith.gemspec
index adbe62a..224ed9e 100644
--- a/flagsmith.gemspec
+++ b/flagsmith.gemspec
@@ -24,8 +24,10 @@ Gem::Specification.new do |spec|
   spec.executables = spec.files.grep(%r{\Aexe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
+  spec.add_development_dependency 'benchmark' # Required by RuboCop; no longer a default gem in Ruby 4.0
   spec.add_development_dependency 'bundler'
   spec.add_development_dependency 'gem-release'
+  spec.add_development_dependency 'ostruct' # Used in specs; no longer a default gem in Ruby 4.0
   spec.add_development_dependency 'pry'
   spec.add_development_dependency 'pry-byebug'
   spec.add_development_dependency 'rake'