I've found some png files in a game. When browsing them with ACDSee, they are 4 colors or 16 colors.

It seems Photoshop only support true color and 256 indexed color png files. Yesterday I tried ACD FotoCanvas. It supports 4bits(16 colors) png file, but unfortunately it does not supports transparent.

I wonder which tool do you use to handle with png files?
Does it support saving transparent png files that less that 256 colors?