After last move,there is no meaningful move left for White.Which move could have been played better?

Maybe 4.d3 should have been 4.d4 and 9.gxf3 should have been 9.Bxf3 ?

I never thought computer will play 8...Nxf3+ since Knight was in stronger position and there was no need to exchange for Black.

thats true?!

I agree with 9.Bxf3. Other than that, I'd just say don't leave so many pawns hanging. By the last move, it's clear that black is winning due to the fact that he has way more pawns left.