Metal Slug 3 was released for PS2 in Japan/Asia but not in US/Europe because Sony wouldn't allow them to release it without double packing it in with another game (similar to KoF). They felt a single 2d game didn't have enough 'value' in today's market.

That's why they ported it over to Xbox and released THAT in the US.

the port of 4 is out in Japan/Asia and it's a totally bare bones port. None of the excellent extras that the port of 3 had. I don't really expect them to do much with 5, so chances are you'll be getting what you pay for when you buy the 4&5 disc.

Metal Slug 4 comes out for Xbox in Japan on Feb 24th according to Play Asia, and Metal Slug 5 doesn't have a release date for any consoles yet.