There are separate cache folders used for the different acts, so if act 4 saves will not load but act 3 is fine, there may be a corrupt file in the act 4 folder interfering with the load process.

Try deleting the files within the dynamic folders (not the folders themselves, though); they will be re-created as required the next time you start the game.
'..\Beyond Divinity\Acts\Act1\Dynamic', '..\Beyond Divinity\Acts\Act2\Dynamic', etc.