I think d5 is the best move, and after Bxd5, Nf6
5. .. d5 6.Bxd5 Nf6? 7.Bxf7+ Ke7 (7. .. Kxf7 8.Qxd8 ) 8.Ba3+ c5 9.Bxc5+ wins at least the Black Queen.
So what are your thoughts on the position? Who is is it worth the pawns?
Why would you play d4?
It is a bit too early to give a definite answer , but I think White surely has something going. If he can castle soon then he is likely to be on top. So yes , I think it might be worth the pawn sacs.
5. .. d5 6.Bxd5 Nf6? 7.Bxf7+ Ke7 (7. .. Kxf7 8.Qxd8 )
7. Bxf7+, Kxf7; 8. Qxd8, Bb4+; 9. Qd2, Bxd2 (off queens and material up, black is better)
Yes you are absolutely right . I missed 9. .. Bb4+! This will make 7.Bxf7+ a bad move so my line doesn't suffice. So now we have 5. .. d5 6.Bxd5 Nf6 and have to start looking from here.
You can play this opening, but is consider not right with good play by black. Still, in a bullet game is not comfortable to play with black and you can probably win many games, but still...