Shameless self advertising, but it's still interesting: https://tcrf.net/Gunforce_II Everything was found by me, unsurprisingly; I'm kind of kicking myself in the head for just now finding this for how much I've played it. The game already runs like shit with one player, so I have no idea how they would have managed with four. However, this same team would go on to release Metal Slug 2, so I don't think they had very high standards for when it came to performance.
Edit (12/3/17): Unused tileset discovered!
Edit (12/3/17): Unused tileset discovered!