Help with Petroff to Four Knights Transposition

A few months back, I chose the Petroff as my defense to 1.e4 2.Nf3.

After 1.e4 e5 2.Nf3 Nf6, my opponents have been playing 3.Nc3, and according to theory, 3...Nc6 is Black's best reply, which transposes the Petroff into the Four Knights.

I don't have any sharp lines worked out in the Four Knights, anything that would present problems for White to solve in a 15 10 game.

Can Black give White a headache in the Four Knights? Are there any sharp replies?

Based on your wording "according to theory, 3...Nc6 is black's best reply" I'm inferring that you don't want unsound suggestions that may offer you practical chances over the board, but may not be correct with best play. 

Probably some of your opponents will go 3.Nc3 because they don't know the Petroff, even though they are not Four Knights players.  So I would recommend the Rubinstein variation 1.e4 e5 2.Nf3 Nf6 3.Nc3 Nc6 4.Bb5 Nd4, which is a tactical line where White can get into trouble if he doesn't know it.  In the main lines Black does at least as well as in the Petroff. 

The Scotch Four Knights with 4.d4 is another matter but the main line should be fine for Black and once again, many players as White won't know it very well. 

You may not like the Guioco Pianissimo, which comes after 4.Bc4 Bc5 5.d3 d6 because you'll need toothpicks to keep your eyelids open.  So you can instead consider 4...Nxe4!? 5.Nxe4 d5.  (But if so, then had better study 5.O-O!? to make sure you don't get in trouble in that line.)

If instead you are like me, then you'd just intensively study something crazy like 3...d5?! and learn all the ways that White can go wrong in that line. (Maybe 4.Nxe5 d4 5.Ne2 Nxe4 with a good game for Black.)  Then you'd be playing an unsound line where White can get an advantage, but you'd win a lot of games against inaccurate play by your opponents who have no idea what to do.    However this approach is not for the faint of heart, so don't try it if you're afraid of losing. 

@ erixoltan and Conzipe - Exactly what I was looking for, ideas tested by experience. I will study them. Thank you.

The line 1.e4 e5 2.Nf3 Nf6 3.Nc3 Nc6 4.Bb5 Nd4 5.Nxd4 exd4 6.Ne2 (instead of the 6.e5 line) picks up the d4 pawn, but don't know how good it is.  NM Jordy Mont-Reynaud was experimenting with that line, but it may not be the best.

An annoying idea against 4...Nd4 is 5.0-0!?, leading to positions where white can play for a win without needing to remember all the theory of the classical lines. Addding this to the very drawish 5.Nxe4 line has always scared me away from 4...Nd4. I play 4...Bb5, the classical line, which is not very sharp in itself but can explode into life later and is a good battleground for outplaying an inferior opponent. 

Against 4.d4 you might want to sharpen the play with Marshall's 4...Bb4!? rather than the solid 4...exd4.

You will ofter meet 4.Bc4?! from players who are bluffing and do not know anything about the four knights. It's really worth studying 4...Nxe5!, it will earn you many points since white is the one which has to be careful to reach equality!

After 4...Nd4, 5.0-0 is annoying. I couldn't find any line where White could go wrong. If White plays 5.Nxd4, even though the evaluation reads (=), the play is complicated.