AT4 and X31 are both anti-tank missiles. They are very similar in many ways, but there are also some key differences.
The AT4 is a shoulder-launched, single-shot missile. It is designed to be used against tanks and other armored vehicles. It can also be used against buildings and fortifications. The X31 is a vehicle-mounted, multi-shot missile. It is designed for use against tanks, armored vehicles, and bunkers.
Both missiles use HEAT (High Explosive Anti Tank) warheads. These warheads are designed to penetrate armor by using a focused blast of energy to create a small hole in the armor plating. The AT4 uses an 84mm warhead, while the X31 uses a 125mm warhead. This gives the X31 a much larger effective range than the AT4 (up to 4km compared to 500m for the AT4).
The main difference between these two missiles is their guidance systems. The AT4 uses a simple optical sight for guidance, while the X31 uses an advanced infrared seeker system. This allows the X31 to engage targets that are moving or hidden behind cover (such as smoke or dust).