Of cause you can still develop like before with your favourite Javsacript engine and JSON. But the developers claim that we application will have the same speed with the new technology.
Rails 4 is automatically threadsafe without the config variable "config.threadsafe". It also includes Live-Streaming for persistent connections.
Some features of the earlier rails version are now available as modules. And can be added as needed.
We already have a migration entry to the Rails 4 RC to migrate your Rails 3 app to Rails 4.