If either target is illegal as the level 1 class ability resolves, no damage will be dealt. If both targets are illegal, the ability won't resolve at all.

Hunter's Talent's level 3 class ability will trigger only once during your end step, no matter how many creatures you control with power 4 or greater. However, if you don't control a creature with power 4 or greater as your end step begins, the ability won't trigger at all. It's not possible to put a creature with power 4 or greater onto the battlefield or increase a creature's power to 4 or greater during the end step in time to have the ability trigger. If you don't control any creatures with power 4 or greater when the ability resolves, the ability won't do anything.

Each Class has five abilities. The three in the major sections of its text box are class abilities. Class abilities can be static, activated, or triggered abilities. The other two are level abilities, one activated ability to advance the Class to level 2 and another to advance the Class to level 3.

Each Class starts with only the first of its three class abilities. As the first level ability resolves, the Class becomes level 2 and gains the second class ability. As the second level ability resolves, the Class becomes level 3 and gains the third class ability.

Gaining a level won't remove abilities that a Class had at a previous level.

Gaining a level is a normal activated ability. It uses the stack and can be responded to.

You can't activate the first level ability of a Class unless that Class is level 1. Similarly, you can't activate the second level ability of a Class unless that Class is level 2.

There's no restriction on how many Class permanents you can control, whether they're the same or different classes. Each Class permanent tracks its own level separately.