I'd suggest reading about the features gained when running it on iP4, as well as the features that you don't get due to hardware limitations.
On the iP4's in my family I chose to leave them on iOS 5.x. They run faster & deliver much better battery life than my friends iP4's on iOS 6.x
My iPhone 5 is fast fun & stable on 6.1, but then again it's designed for it.