Is 4GB of RAM enough for a new Retina MBP?
No it is not enough to have 4GB, since it is minimum you can have and you will always run at max of it.

Mavericks will do the best to manage the space, but there is a limit what it can do.

Big RAM users (aside of the OS) are the web browsers.

It is all dependent what is your activity, apps you use. As said Mavericks will do great job in working your RAM to keep it to a workable levels, but adding RAM can only help.

Here is an example from my MBA with 4GB.



And the top 10 totaling 1.5GB  (as you can see I do not run RAM heavy apps but do get over 4GB.



Yes, do the RAM upgrade, it is worth the money.