4...dxe5 5 Nxe5 Nd7 6 Nxf7 is a sacrifice white can try leading to 6...Kxf7 7 Qh5 Ke6
Alternatively, 5...c6 secures the knight so the sacrifice would not be dangerous.
Alekhine defense: help needed!
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This is probably the most precise way to counter the alekhine. With Nf3 w delays the c2-c4 push, depriving B of an important source of counterplay (the weakened d4 pawn).
B has developed several tryes during the years. He is not lost at all but has trouble getting an inspiring game.
-the modern 4...exd5 is possibly the best move, and i have always played this as B. B relies on the fact that 5.exd5 Bg4 is ok for him. So W is forced to play Nxe5, losing his crampling e5 pawn and somewhat freeing B position. Another point is that B will soon or later play Nd7 challenging W seemingly strong knight, when Nxd7 concedes B equality (by exchanging a piece B has solved his only problem, the lack of space) and Nf3 loses a tempo.
The bad side of the coin is that once the e5 pawn is gone B position is solid but rather passive an not alekhine-like. B usually hopes to reach a sharp unbalanced game where he tries to prove the e5 pawn overextended. Here the e5 pawn is gone and B is simply slightly behind in space with little counterplay in a somewhat scandinavian-feeling position. He is very close to full equality, but the positon is not very exciting.
There are 3 main variations after 4...dxe5 5.Nxe5:
-...Nd7 is a sharp move, trying to lure W in the unclear sac 6.Nxf7. B needs to study quite a lot of lines to survive. Another problem is that W can simply play 6.Nf3, trasposing to 5...c6 lines. For this practical reason i don't like Nd7.
-...g6 is the kengis variation. Studying the games of GM Kengis will give you an idea of the position. 6.Bc4 is probably ok for B, but c4 and the ultrasharp Qf3 are move worring. Overall the variation is very playable if B knows what he is doing. The kengis requires quite a lot of strategical understanding, and in my personal experience B position tend to slowly collapse when handled by an average club player.
-...c6(miles variation) looks slow but is in fact a clever waiting move. I have scored very well with this in the past. B is now ready to meet 6.c4 with ...Nb4! (thanks to the fact that Qa4 is not check anymore) and Bc4 with g6(kengis by trasposition) or something like 6...Nd7 7.Nf3 N7b6 8.Bb3 Bg4 with a nice position.
W usually settles for a quiet developing move like Be2 or Bd3, ad B follows up with either Bf5 or Nd7 with a solid position. A sharp line is 6.Be2 Bf5 7.g4!? but B is supposed to hold his own in the complications.
As a whole dxe5 is a good move and can be played with little theory if followed with c6 but if you like to get sharp positions from the opening you have to look elsewhere.
The old 4...Bg4 is an alternative which i have never played but i'd like to try it one day or another. 4...g6(Alburt variation) is sharp but demanding and possibly is under slight theoretical trouble. 4...Nc6 is a provocative move. B gets an interesting game if W goes for the "refutation" 5.c4 Nb6 6.e6!?, but unfortunalely 6.exd6 exd6 7.d5 Ne5 8.h3 is quite unpleasant to play against.
it's nice to talk with another alekhine lover, we are so rare 
In fact at the moment i'm not playing the alekhine. I used to score very well with it but i slowly came to the conclusion that it's essential for your chess development to play 1.e4 e5 as B for a while.
What do you intend to play against the other W tryes?
I used to play 1.e4 Nf6 2.e5 Ne5 3.d4 d6 4.c4 Nb6 5.f4 dxe5 6.fxe5 c5!? against the four pawn attack which great results on another website, i don't know if the variation is still considered sound but play is ofter sharp and thematic and i scored close to 100% in quite a few correspondence games.
Against the exchange i hope you are going for cxd6! I would drop the whole defense rather than playing exd6. B is left with a passive position, certainly defendable but depressing.... cxd6 is much more interesting and you shouldn't be scared by the popular voronzeth variation. B can try a few interesting moves and usually archieves a very playable position. Only a player with a great endgame technique can prove something as W in this variation.
Bg4 is not passive at all in my view. It's true that W has the initiative for a long time but B has good long term prospects. Also the flohr variation(bg4+c6) leads to different structures where play is quite thematic. And the variations are difficult for both sides.
I agree that the provocative Nd7 is not practical since it can be easily move-ordered, but the comparison with the latvian is a bit harsh in my opinion. It's not clear if W has any advantage after the sac.
...Nc6 is not a good idea in my view but not for the reason you mentioned. after e6 it's true that W has several options but many of these lead to unclear positions which are very dangerous for both sides. In my view W position is often the most difficult to play. I think Nc6 would be a lot commoner if only W was forced into playing the pawn sac. Unfortunately W has a simple but annoying idea: he can just traspose to an exchange exd6 variation where B is under slight but persistent pressure.
does anyone know any ideas/analysis/traps/words of wisdom on the malekhine, modern variation?