how do u unlock the 4 mission

Do you mean how to finish mission 4 or how to finish mission 3 so you can access mission 4?

To finish mission 3 shoot down all the red plane, destroy the 2 tanks (shot them form far away and don't fly straight, they fire shells like rockets) and bomb the hangars.

mission 4 just shoot all the red planes coming to your base. hit them as soon as you see them, they make fast turns and are hard to get if they get past you.