Something obviously made you choose a S4 in the first place, so I am presuming you find Android a viable alternative, have some interest in a larger screen phone, and aren't so locked down by the iOS ecosystem that you absolutely need every device to be an Apple device.
As such, I'd keep the S4 - because at this particular time I think Android/Samsung have comparatively more exciting stuff on offer. The 5S is just another S upgrade, nothing really WOW will be happening with it. And you get to use iOS7 on your iPad anyway.
So you could take the opportunity to get to know the S4 and Android, play around with them and see what they can do. Then, next year - that'd be the time to look out for a really exciting iPhone, methinks. Then you could sell the S4, probably at a decent used rate, and get a new, reinvented iPhone.
If you sell the S4 already, chances are you won't even know what you missed. - I got my first Android phone this spring and I am still discovering new stuff to do with it, though it took me a few weeks to really understand what a powerhouse it is. My Note 2 is actually
fun, as opposed to just useful like the iPhone was for me. But though I'm not very excited about the new iPhones coming this fall, I'll sure as hell have my eyes open for what comes next year. And I'd love nothing more than to be blown away by the iPhone 6. Right now though - this is Android's year, if you ask me.
All subjective advice of course, but you asked for it