Simply put, 4G is what comes after 3G. Today's wireless broadband networks are powered by 3G - short for third-generation -- technology. In the U.S., that means HSDPA/UMTS and CMDA EVDO radio technologies. These are currently used by AT&T/T-Mobile and Sprint/Verizon, respectively. The 3G standard was devised by the 3GPP2 organization. The same and other standards bodies are working to finalize what 4G -- or fourth generation -- wireless networks will include. The standard has not been finalized.
Let me repeat that: The standard has not been finalized. Without a finalized standard, technically speaking, 4G hasn't been defined and doesn't exist. Any company claiming to offer 4G products and/or services is not telling the entire truth (hell, it might even be misleading consumers).
Fourth generation wireless networks are to be IP-based, and must meet the IMT (International Mobile Telecommunications) Advanced criteria.