Trade-in

You can trade ONE device towards the purchase of another device. But you can trade in multiple other devices in return for an Apple Gift Card. I would trade the 3 phones in for Gift Card. Then use the last phone as your trade and then for payment use the Gift Cards. The devices you trade must work and not have Activation Lock on them. If they are damaged or can't be prepared correctly as in wiped to factory settings, they will have no value.

See this link on what to do before you sell or trade your iPhones --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Hello,

Yes, if the phones have value, you should be able to get a store credit. See this:

Apple Trade In - Apple

Good luck.