It seems like a great deal of people have not bought this game yet.
READ MY LIPS
BUY CONTRA 4
YOU KNOW YOU WANT TO
*ahem*
It has a main game, and just TONS of extras. Unlockable Characters, promotional comics, and best of all, Classic Contra games. Of course, you're going to have to play HARD for these goodies- but hey, it's a labor of love if you're into Contra!
The only thing truly lacking is an online co-op mode, but you're nuts if you passover this game for solely that reason.