Rails 4.0 and newer version will support ruby 1.9.3+ only. It wont work with lessor version then ruby 1.9.3. Major developments are below –

Ruby 2.0 preferred; 1.9.3+ required

Strong Parameters

Turbolinks

Russian Doll Caching

Live Streaming

Rails Queue

Asynchronous Action Mailer

Barebone Models – ActiveModel::Model

Http PATCH

Custom Flash Types

Flush content_for blocks

Rounting Concerns

Multiple Route Files

PostgreSQL Array Support

Page and Action Caching

Cache Digest

Controller Wise ETags

where.not and Chainable relation.none and Relation._!

Action Callbacks Renamed

Default Index Page

HTML5 Input Helpers

Models Concerns

vendor/plugins and Ruby 1.9.3 Lesser Banned!