First, I will describe 3G. 3G involves the use of broadband and packet data. The 4G technology involves broadband, packet data, and all IP, which makes your phone function better on the Internet.
Also, the 4G network has more bandwidth to transfer data. Here are the stats.
500-700 kbps voice
3.1 mbps data
100-300 mbps voice
3-5 mbps data