Dude, as far as Im concerned.......the game only came out in Japan.
In PAL and NTSC-U territories, they stopped at 3, so that is a repro cart that might have been made to look a little distressed to give the impression that is actually an official release.
Can someone verify whether 4 came out in the US? as I am 99% sure it was Japan only and a quick googling says the same........it would obviously answer why there is no horizontal retail cover for it as well.

Up to you though man, just wasnt sure you were into repro's.