4G refers to the 4th Generation of mobile technology. First generation (1G) was the analogue standard deployed by mobile networks launched in the early 1980's - Nordic Mobile Telephone (NMT) in Europe and Advanced Mobile Phone System (AMPS) in the US.
Second generation (2G) was the digital standard utilised by mobile networks launched in the early 1990's (GSM around the world, TDMA/CDMA in the US).
Third generation (3G) was launched in the early 2000's with WCDMA and EVDO as the main standards.
4G is the fourth generation, and was first deployed in networks around 2008-2009.. 4G today mainly refers to Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMax) or (High Speed Packet Access (HSPA+)