How to switch my iphone 4 from Verizon to Straight talk?

I was given a phone from a friend who upgraded to a newer phone. Her service on the iphone 4 that I now have is Verizon. I would like to switch that service and use Straight Talk instead. How may I do this?

This web page at Straight Talk will let you check if your phone is compatible:
http://stbyop.com/straighttalk/

My guess is no, because these days, Straight Talk's "bring your own phone" setup requires a SIM card, but the Verizon iPhone 4 doesn't have a SIM slot.

You have to unlock it first

Probably not. If you want to use the iPhone on a Pay-as-you-go plan (not a bad idea), then you have to use a Verizon MNVO like Page Plus Cellular. FWIW: Wikipedia has a list of North American MNVOs.

You have to buy a code from Straight Talk at Wal-Mart for $15, after you have made sure that the phone you want to use is eligible to be used with Straight Talk. Then you activate the phone with the code and buy a $45 Straight Talk airtime card for service.