As of February 2015, the fastest 40-yard dash that has been officially recorded is 4.24 seconds. The time was set by Rondel Melendez and Chris Johnson at the 1999 and 2008 NFL Pre-Draft Combines, respectively.
The 40-yard dash isn't an official track and field event. It is used by NFL scouts to judge the speed of prospective players. Additionally, the NFL has only kept official records of the 40-yard dash since 1999, when electronic timing was first used. Before that, Bo Jackson and Deion Sanders were recorded running the dash in 4.12 and 4.19 seconds respectively, but were hand-timed, making the times unreliable.