4X (fourcross) is the newest and arguably most spectacular form of mountain bike racing.

It is also known as many other things, most commonly Mountaincross (mtx), but also as Bikercross (bx), Bicycle Supercross (bsx), and more.

It first began on the World Cup circuit in 2001, replacing the two-man "Dual Compressor/ Slalom". From there it spread throughout the Mountain Biking world.

4X is a blend of Motocross, BMX, and Downhill Mountain Biking. It involves four mountain bikers racing head to head on a downhill “Supercross” style track.

Riders start on a start ramp with a BMX style start gate. The course usually consists of straights with jumps and other features of various sizes and styles, and corners that are usually bermed but sometimes can have no berm.

A 4X course is typically up to 6 metres wide and between 250 - 500 metres long. Race times vary from 25 seconds up to 50 seconds.

The fastest riders use either hardtail or short travel dual suspension bikes.