The only thing the 4 really needed was speed. I used one for a long time. It is pitifully slow.
I bought a 4s and THAT is a huge upgrade already.
Yeah. I got the 4 when it was first released. I still remember how blazingly quick it was on iOS 4. It was my first ever iPhone and I really did love it. Now it's unusable on the latest OS.
OP, I had a 5, now a 5S, and I can assure you that the performance difference is out of this world. There's absolutely nothing the 5S can't handle, its CPU is ridiculously good. And this is coming from somebody who thought the 5 was a little slow at times (it really isn't).
Just to emphasise: I can't keep up with my 5S. The speed is ridiculous.