Go with Rails 4.
Rails is fast moving and Rails 5 will most likely be out by the time you have finished your project.
Actually, Rails 4 has many cool new features. Not least of which is live streaming. The beggning of a project is a good time to learn new stuff.
What you want and what you need are too often not the same thing!
When your head is hurting from trying to solve a problem, stop standing on it. When you are the right way up you will see the problem differently and you just might find the solution.
(Quote by me 15th July 2009)