Rails 3 to Rails 4 upgrade explained.

application.rb.diff

-Bundler.require(:default, Rails.env) if defined?(Bundler)
-if defined?(Bundler)
-  Bundler.require(*Rails.groups(:assets => %w(development test)))
-end
+Bundler.require(:default, Rails.env)

environments_production.rb.diff

-  # Enable threaded mode
-  # config.threadsafe!
+  # Eager load code on boot. This eager loads most of Rails and
+  # your application in memory, allowing both thread web servers
+  # and those relying on copy on write to perform better.
+  # Rake tasks automatically ignore this option for performance.
+  config.eager_load = true

Gemfile.diff

diff --git a/Gemfile b/Gemfile
index 084537c..0a99d96 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,12 +1,12 @@
 source 'http://rubygems.org'

-gem 'rails', '3.2.8'
+gem 'rails', '4.0.1'
 gem 'sqlite3'

 # asset pipeline
 group :assets do
-  gem 'sass-rails',   '~> 3.2.3'
-  gem 'coffee-rails', '~> 3.2.1'
+  gem 'sass-rails',   '~> 4.0.1'
+  gem 'coffee-rails', '~> 4.0.1'
   gem 'uglifier',     '>= 1.0.3'
 end

Gemfile.lock.diff

-    activesupport (3.2.8)
-      i18n (~> 0.6)
-      multi_json (~> 1.0)
+    activesupport (4.0.1)
+      i18n (~> 0.6, >= 0.6.4)
+      minitest (~> 4.2)
+      multi_json (~> 1.3)
+      thread_safe (~> 0.1)
+      tzinfo (~> 0.3.37)