44 ( +1 | -1 ) The move hardly deserves ?!, and it's not really a gambit since white gets the pawn back fairly easily. Estrin's book is out of print, so you might have a hard time finding a copy, but, to my knowledge, the main line Max Lange Attack runs 4... exd4 5. 0-0 Bc5 6. e5 d5 7. exf6 dxc4 8. Re1+ Be6 9. Ng5 Qd5 10. Nc3 Qf5 11. Nce4 0-0-0 12. g4 Qe5 13. fxg7 Rhg8 14. Nxe6 fxe6 15. Bh6 d3 16. c3 d2 17. Re2 Rd3=. There are several books on the Max Lange and the other variants if you want to learn more.
18 ( +1 | -1 ) Better,in my opinion, 5...Nxe4; anyway Black gets an equal game, but no more, while in the 4.Ng5 variation he has interesting chances for a pawn. So, if you want to mark 4.d4 you should mark it "!" :)