Really, the most important song is the first one. You need a hook that will grab the listener's attention right away. Then the next two songs need to also be great to keep their attention.
I think the tendency is for the fourth song to be very different - often much softer - than the first 3. That's probably something to be wary of since it can set the tone for the rest of the album. Listeners might feel like it's a bait-and-switch and sour on the rest of the album.
Then, of course, the last song should *actually* be the best song on the album, but unmarketable for some reason.