3G is generally talk, text, and 3G basic internet. While 4G comprises of 3G & 4G high-speed internet for multimedia, apps, and data-heavy streaming.

So a 3G booster will only do talk, text, and basic 3G data, while a 4G booster will do talk, text, 3G & 4G data. It's recommended to go with a 4G booster since it is more future-proof.