Is there something in Rails 4 that you desperately need? If not, wait. This looks like it will be a smother release than 2.x to 3.x was, but I wouldn't add this

Is there something in Rails 4 that you desperately need? If not, wait. This looks like it will be a smother release than 2.x to 3.x was, but I wouldn't add this variable to the whole process this late in the game. In the meantime you can watch bug / experience reports and get some info to make an educated decision.