diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 033769a..72851ea 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,9 +13,11 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - 3.1
-          - 3.2
-          - 3.3
+          - "3.1"
+          - "3.2"
+          - "3.3"
+          - "3.4"
+          - "4.0"
       fail-fast: false
     steps:
       - uses: actions/checkout@v4
@@ -32,11 +34,11 @@ jobs:
       - run: bundle exec standardrb
       - name: Run tests
         env:
-          COVERAGE: ${{ matrix.ruby == '3.3' && '1' || '0' }}
-          TEST_NODE_PARITY: ${{ matrix.ruby == '3.3' && '1' || '0' }}
+          COVERAGE: ${{ matrix.ruby == '4.0' && '1' || '0' }}
+          TEST_NODE_PARITY: ${{ matrix.ruby == '4.0' && '1' || '0' }}
         run: bundle exec rake test
       - name: Upload coverage to Codecov
-        if: matrix.ruby == '3.3'
+        if: matrix.ruby == '4.0'
         uses: codecov/codecov-action@fb8b3582c8e4def4969c97caa2f19720cb33a72f
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
@@ -57,7 +59,7 @@ jobs:
           lfs: true
       - uses: ruby/setup-ruby@afeafc3d1ab54a631816aba4c914a0081c12ff2f
         with:
-          ruby-version: 3.3
+          ruby-version: "4.0"
           bundler-cache: true
       - name: Ensure e2e credentials are configured
         if: ${{ env.TRANSLOADIT_KEY == '' || env.TRANSLOADIT_SECRET == '' }}
diff --git a/.gitignore b/.gitignore
index 8a2cdcc..166ed01 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,4 +16,4 @@ env.sh
 .env
 vendor/bundle/
 .docker-cache/
-.cache/rubocop_cache/04e06e0faf5ad652d8bcbcfd85bac5f6c32e711e/3031a80880d8a984708138f0d003f77c4bad2648
+.cache/
diff --git a/.standard.yml b/.standard.yml
index e69de29..cbce7d3 100644
--- a/.standard.yml
+++ b/.standard.yml
@@ -0,0 +1,2 @@
+ignore:
+  - ".docker-cache/**/*"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 684df40..340926a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,8 +1,10 @@
-### 4.0.0 / 2026-09-02
+### 4.0.0 / 2026-09-03
 
 - BREAKING: Replace the long-dormant `rest-client` transport with Faraday 2 and `faraday-multipart`.
+- BREAKING: Raise the minimum supported Ruby version from 3.0 to 3.1.
 - Keep HTTP client types out of SDK responses and exceptions while preserving response status, headers, body access, multipart ordering, and rate-limit retries.
 - Add a [v3 to v4 migration guide](MIGRATING_TO_V4.md).
+- Test all Ruby versions from the supported minimum through Ruby 4.0.
 
 ### 3.1.2 / 2025-10-29
 
diff --git a/Dockerfile b/Dockerfile
index 8f85b91..26f9631 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1
 
-FROM ruby:3.3 AS base
+FROM ruby:4.0 AS base
 
 RUN apt-get update \
     && apt-get install -y --no-install-recommends \
diff --git a/MIGRATING_TO_V4.md b/MIGRATING_TO_V4.md
index 17f73ea..c8698ab 100644
--- a/MIGRATING_TO_V4.md
+++ b/MIGRATING_TO_V4.md
@@ -5,7 +5,8 @@ Transloadit API exposed by this gem and do not need code changes. The migration
 because v3 also delegated undocumented methods to RestClient response objects and exposed
 RestClient exception inheritance.
 
-The minimum supported Ruby version remains 3.1.
+Version 4 requires Ruby 3.1 or newer. The final v3 release allowed Ruby 3.0, so applications still
+running Ruby 3.0 must upgrade Ruby before updating this gem.
 
 ## Update the dependency
 
diff --git a/lib/transloadit.rb b/lib/transloadit.rb
index 9f5c9d3..a9f6f1f 100644
--- a/lib/transloadit.rb
+++ b/lib/transloadit.rb
@@ -2,7 +2,7 @@
 require "date"
 require "json"
 require "openssl"
-require "cgi"
+require "cgi/escape"
 require "uri"
 
 #
diff --git a/transloadit.gemspec b/transloadit.gemspec
index 12c3d06..afcbef7 100644
--- a/transloadit.gemspec
+++ b/transloadit.gemspec
@@ -29,9 +29,9 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency "rake"
   gem.add_development_dependency "minitest"
   gem.add_development_dependency "simplecov"
-  gem.add_development_dependency "standard"
+  gem.add_development_dependency "standard", ">= 1.54"
 
-  gem.add_development_dependency "vcr"
+  gem.add_development_dependency "vcr", ">= 6.4"
   gem.add_development_dependency "webmock"
 
   gem.add_development_dependency "yard"