If I pay for Chapter 4 will I have to pay for all future chapters?!
I'm sorry you are having problems with the save.
If your savegame was from the -public edition- then that could be a problem but I would have to test that
You could try deleting the "persistent" file from the game's save folder and from the windows temp renpy folder->Game's name->Saves
Maybe send me your traceback.txt file and the save file so I can check it (itchv19games@gmail.com). But at that point I think is better for you just to start a new game and use the -ctrl- button with the "skip unread" option marked in the options until you reach the new content.