What iPhone 4/4s can use for Walmart family mobile plan?

I want to buy a used iPhone 4 or 4s or 5 but I don't know what it needs for me I be able to use it for Walmart plan. How do I know if it will take a SIM card? Does it have to be unlocked? Help me out please! I'm looking for one on amazon

Buy an unlocked one or unlock a locked one. For unlocking you would need to know the original carrier of the phone as the unlock price depends directly on which carrier it was bought with.

Your phone has to be unlocked in order to use it with that SIM.

I hope that replied your question. You can always ask for an IMEI number of the phone before buying and check if it is unlocked / locked and to which carrier it is locked.