I play both. Depends on the mood. The Modern variation when I'm in my solid frame of mind (not often in fast time controls) and the Four Pawns when I'm in a frisky mood. I would definitely look at both.
Against the Ne4 and Ng8 lines, don't worry, play common sense chess and you'll be fine/better. Look at the first so you have an idea of what's going on, but I'd stamp it as utter
junk.
In the Modern variation, where black exchanges on move 4 (and in most cases when he does this at a later point) I know that there is theory around that'll say that the pawn recapture is best. You will want to recapture with a knight as a general rule. If the pawn goes up there it's a sitting duck.
There are all sorts of tricky lines, all following the main Alekhine defense (very modern btw) idea of undermining the center.
Here is a fascinating game between Bronstein and Ljubojewic in the Four Pawns. The line with c5 is really inventive and razorsharp.