Originally Posted by WIRunner

The iPhone 4s gained a CDMA/GSM chipset, that allowed for a SIM card for Sprint or Verizon. US Cellular (the other "major" CDMA carrier) did not use a SIM in the 4s, but left the GSM side unlocked on that device, it was carrier locked on the 5c and up though, and unlocked for international SIMs on all devices. Verizon is required, as part of their 700 MHz purchases a few years ago, to have all LTE phones fully unlocked.