Well, here's the deal with that....Miki 4 and Miki 3 share the exact same UV map. Most likely, the somewhat more realistic shape of Miki 4's stomach/navel area may have exposed parts that were enclosed in Miki 3's simple belly button so that textures that discolored the belly button area are more noticeable.  It's not really a problem with the textures, so much as it is the fact they weren't specifically made for Miki 4.