Opening assistance for a beginner I am new to chess and am confused on what to do with the following opening

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1. e4 e5
2. Nf3 Nf6
3. Nc3 Bb4


I have been playing 4. a3, but the Bishop then proceeds to take c3 so I end up with a doubled pawn.

If someone could offer some guidance or tell me what this opening is called so I can do some research, it would be very much appreciated.

Cheers
Edit: Using the chessgames Opening Explorer, I confirmed the move is playable, here is the list of possible replays:

Move     #games
---------------
4.Nxe5   175
4.Bc4    151
4.d3     52
4.a3     14
4.Nd5    12 &lt;- my option
4.Be2    2
4.d4     2
4.Bd3    1
4.g3     1


4.Nd5 looks viable and easily prevents that double pawn.

It also pushes the game into an aggressive one, which can be good or bad depending on your play-style.

Final position after 4.Nd5:

rnbqk2r/pppp1ppp/5n2/3Np3/1b2P3/5N2/PPPP1PPP/R1BQKB1R b KQkq - 5 4


He will most likely do NxN, I don't see many other choices here as both the Bishop and the e Pawn are attacked:


4...Nxd5
4...Bd6 (weak move)
4...a5 (weak move)