Act 3 exploration is the 1st story content that give you a 4 star.
Other than that, you can try the premium hero crystals.
Also, once you have around 21 three stars at decent levels you can use them to grind for the 4star arena if you are still in the beginner bracket. I heard the last beginner cutoff was very very low for AA.
Dupe a lot of 3*s/ Join a better alliance/ Play arenas.. I feel your pain.. I think I was level 34 before I got my first 4* (Punisher from arena crystal) I joined a better alliance then they started flooding in after that
I was 6 months into the game and level 37 when i got my first 4 star. Its slightly easier now. Hang in there. Dont worry, rest, as some one above said. Play with a good alliance.
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Other than that, you can try the premium hero crystals.
Also, once you have around 21 three stars at decent levels you can use them to grind for the 4star arena if you are still in the beginner bracket. I heard the last beginner cutoff was very very low for AA.
A time machine to yesterday and $30 ought to do it.