3G and 4G are different generations of mobile-network technology: 3G is older, while 4G LTE is faster and supports richer data services. The Federal Communications Commission says U.S. carriers have phased out or retired 3G networks, and some older 4G phones may also lose service if they do not support Voice over LTE (VoLTE). In practice, a phone that supports 4G LTE with VoLTE is the safer choice; check with your carrier for device and coverage details.
Primary source: Federal Communications Commission: Plan Ahead for Phase Out of 3G Cellular Networks and Service