I bought an iPhone 4 but i don't know if I should keep it?

I thought I had to return my iPhone 5 back to Verizon so I bought an iPhone 4 just so I could get buy and then buy the 6 but it turns out I don't need to return my iPhone and I'm wondering if I should keep it just in case, sell it, or return it?

If you don't want it or need it may as well return it.

Return it, the iPhone 4 is really old and outdated, so why risk trying to sell it. Return it if you can, cheers.

Sell the one you don't need, no reason to hold and let it depreciate, sell it while its worth something. You also didn't say if the other phone is now blacklisted or not, obviously the value goes to almost 0 in that case.

Sell and Buy Android
iPhone 4 won't run any new apps that you use unless you update software but it'll make your old device slow