4G stands for 'fourth generation' and, somewhat confusingly, is a labelled applied to both hardware and also mobile phone radio technologies.
4G mobile phone networks offer superfast data download speeds - up to 10x faster than current 3G networks. In theory 4G download speeds could be as high as 100 Mbit/s.
Examples of 4G networks are LTE, LTE-Advanced and Mobile WiMAX.
In hardware terms, the most popular usage of 4G is in tandem with the Apple iPhone, where it refers to the fourth generation of the handset.
The so-called 'iPhone 4G' is due to be unveiled in the Summer of 2010 - confusingly, it is not yet clear whether the iPhone 4G will actually be 4G-compatible.