Alright, it's actually very easy to unloack the fourth door. See my post below about unlocking the horses. You use the same method, but this time you unlock Mathieus door in the yourname_locks file. You can go in and open the door and see....nothing. Yep, that's right, it looks like the company didn't design for this door to be opened so they didn't create any graphics or animations for inside. Your screen just goes black.