I'm not sure if anyone else has posted about this yet (I searched the thread but didn't find anything). I decompiled the game and opened it up in the WOLF editor, and as far as I can tell, there is no 4th ending, at least in this version of the game. The 4th painting has no secondary event page like the other ones where it would display the image related to the ending. The event code only contains the text "A blank canvas decorates the wall." I've gone through all of the common events, as well as most of the events on important maps, like the king's castle, Alma's room, the mansion, etc.
There is an image called EX_EndingEX_Cut1.png that shows Alma making a smoochy face that says "Thank you for playing, I love you!" but I don't see how that's triggered anywhere.
Also, I looked at the events for the 3 "Shadow Soul" scenes that under the Others tab in the gallery. They are apparently scenes with Matra, Eluna (A character I somehow missed entirely), and Masamune that I missed? Does anyone know how to trigger those? From the text, it seems like you give them a temporary body in a doll.