The Gunsberg Variation

Hello guys! Today I will show you an interesting alternative to the theory-heavy Ruy Lopez, the Scotch and the Italian Game. It's called the Gunsberg Variation and it's a sideline in the Four Knights.
1 e4 e5 2 Nf3 Nc6 3 Nc3 Nf6 4 a3!?

Now what's that move about? It looks like a total beginner's move. It does, however, come with a big perk which also happens to be the whole point behind the move:

It rules out any Bb4, which is a totally adequate reply to 4 d4. That way, d4 is sort of a threat now a3 has been inserted and Black needs to find an answer. Here are the four minor options:

The most challenging reply is the following:

All in all you can learn the theory in less than an hour and in all of the resulting positions you're still at least even. Try it!