So it seems bow went back to pre-World gameplay, people can correct me if I'm wrong. Well in short there has always been charge levels even in world. You know how we would dash between shots to preserve a charge? We were preserving bow charge levels.

Anyway to answer your question, you want to hold fire button to charge the now before letting the arrows fly. The higher level the charge the more damage is does. Rapid shots are your normal arrow attacks. You shoot and they go. Spread type bows are shotgun type blasts. You shoot multiple arrows in a horizontal arc. However, spread shots do not go as far as rapid shots. So they are more close range bows. Finally we got pierce type bows, these can fire the furthest away and their arrow will travel through a monster dealing damage as it goes.

So what does this mean about charge level 4? Well remember the higher the charge the more damage it does. So a level 4 charge does more damage than a bow's level 3 charge. The only way to unlock it is using the Mighty Bow skills. So far in Rise that comes from Arena quests doing the 4* and 5* Arena quests will let you craft a helmet that has it. Do note on some bows a 4th level charge will change the shot type completely. What is normally a rapid type bow at level 3 charge may now become a spread shot.

If bow is truly back to pre-world gameplay then I dont think Bow is going to be an elemental weapon like it was in World. Before World it was your normal raw weapon. Heck back in MHGU the game right before World we used the Nercsylla bow which was a raw, rapid shot, high crit bow.