The simple answer is no, you don't need four piston brakes on your mountain bike. However, there are some advantages to having them that might make them worth the extra cost for some riders.
Four piston brakes tend to have more stopping power than two piston brakes. This can be especially helpful if you ride in hilly or mountainous terrain where you need to be able to stop quickly and safely. Four piston brakes also tend to dissipate heat better than two piston brakes, so they're less likely to overheat and fail during long descents.
Ultimately, whether or not four piston brakes are worth the extra cost is a personal decision. If you feel like you need the extra stopping power and heat dissipation they offer, then they're probably worth it for you. But if you're happy with your two piston brakes, there's no need to upgrade.