Welcome back!
I brought down a used iPhone 4s for a friend and he loves it. I actually use one myself on Digicel in Jamaica and it works great. Just make sure that it is an unlocked GSM iPhone and you will have no problem. If they are your old phones from AT&T or T-Mobile you may have to call their tech support to help you unlock them.
At some point, Apple started shipping all iPhones unlocked - even if purchased from a specific carrier - but I'm not sure what version that was. It might have been the 5 or 6. I purchased my 4s-es from ebay, and specifically looked for unlocked GSM models (CDMA or Verizon models will not work on Digicel). You can find a 4s today for less than $50. However, a 5s is probably the best choice as they still receive software updates from Apple.