I second PagePlus. You cannot use an ATT device on a Verizon network, and vise versa, because the internal hardware is different.
You shouldn't have any problem using your Verizon 4S on PagePlus, though. This is what my sister did a few months back and had no trouble. Check the guide recommended by IP Daley, and also check out kittywireless, which is a service front for PagePlus and can help get everything squared away for you (though they probably wont want to help with your iPhone, but its not that hard).