This subject has been covered before, but maybe it is a good idea to take a look at it with fresh eyes and with comrade Rybka 3  

1.e4 e6 2.d4 d5 3.Nd2 Be7 4.e5 Nh6!? Normal is 4...c5 5.Qg4 and then either ...g6, ...g5?! (6.Qh5!) ...Kf8 or ...Bf8.



I used John Watson, Play the French (third edition; London 2004) as a source since he is the only writer I know who covers 4...Nh6 (maybe Psakhis also, but I didn't check this). White can play:

A.  5. Ngf3 (Not very critical, black can continue according to plan.) f6 6. exf6 Bxf6 7.Nb3 Nf7 8. Bd3 O-O 9. O-O Nc6 10. Bf4 Be7 (10... g5 11. Be3! (11. Bg3 g4 12. Ne5 Bxe5 13. dxe5 Ncxe5 14. Qe2 Qg5 (and white has compensation according to Watson. After 11.Be3! the black position is a little loose.)) 11.c3 Bd6 12. Bg3 Qf6 13. Bc2 b6 14. Qd3 Qh6 15. Rfe1 Bd7 16. Re2 Ne7 17. Rae1 Ng6 18. Nbd2 Nf4 19. Bxf4 Bxf4 20. Nf1 Rae8 21. Ng3 g6 22. b3 Qg7 23. c4 c6 24. a3 dxc4 25. bxc4 e5 26. dxe5 Nxe5 27. Nxe5 Rxe5 28. Rxe5 Bxe5 and finally black has liberated himself. Of course this variation isn't forced but it gives an impression of the black chances.

B. 5. Bd3 f6 6. Qh5+(6. exf6 Bxf6 7. Ngf3 Nf7 8. Nb3 Nc6 see variation A.) 6... Nf7 7. Bxh7?? Nd7 and 8...Nf8 and wins. 

C. 5. Ndf3 c5 6. Bxh6 (6. Bd3 Nc6 7. Bxh6 gxh6 8. Qd2 Qb6 9. O-O-O Bd7 10. Qxh6 c4 11. Qg7 O-O-O 12. Bxh7 Nb4 13. Kb1 c3 14. b3 Nxa2! and black wins (Watson).) 6... gxh6 7. Qd2 Nc6 8. Ne2! (Watson gives only 8.Qxh6?!) 8... Bd7 9. Qxh6 Qa5+ 10. c3 b5!  11. Qg7 Rf8 12. Qxh7 (12. Nc1 cxd4 13.Nb3 Qb6 14. cxd4 a5 with full compensation.) 12... b4 13. Qc2 bxc3 14. bxc3 cxd4 15. Nexd4 Nxd4 16. Nxd4 f6 17. Be2 fxe5 18. Nb3 Qa4 19. Bh5+ Kd8 20. O-O Qh4 with an extremely unclear position.

D. 5. Nb3!(Critical because it threatens 6.Bxh6 and prevents  ...c5.) Nf5 6. Nf3 b6! 7.g4! (7. Bd3 Ba6=) 7... Nh4 8. Nxh4 Bxh4 9. g5 Bxg5! 10. Qg4 Bxc1 11. Qxg7 Rf8 12. Rxc1 (12. Nxc1 c5 13. c3 cxd4 14. cxd4 Qh4 15. Ne2 Nc6 16. O-O-O Qxf2 17. Qg3 Qxg3 18. hxg3 Ba6 19. Rxh7 O-O-O=) 12... Bb7 13. Qxh7 Nd7 14. Bd3 Qe7 with compensation.

This is truly an interesting variation for independent research.