If I brought a 4S to Verizon, locked to another carrier (but the contract is over), would Verizon unlock it to use on their network for a fee? I couldn't find the details on this originally when I checked their web site.
Verizon has no authority or ability to unlock a device locked to a different carrier. And with the 4S specifically, unlocking for domestic use isn't really a thing
If you already have a working Verizon SIM, you can actually use GSM unlocked iPhone 6 and 6 Plus on Verizon (CDMA radio seems to be enabled on those). Only issue is they don't allow activation of service on devices with unknown ESN/MEID/IMEI.