I've taken more than a few NES pads apart and I'm certain they're 4 way, since they only have 4 contacts for the D-Pad. The diagonals come from more than one direction registering simultaneously. A true 8 way would have 8 contacts.
Actually, they way the controller electronics work, you can construct one that will allow you to push all 4 directions at once if you wanted. TASers do crazy things like that, pushing up and down at the same time to, say, launch themselves up ladders in SMB2.