With the latest
patch, the key in the golden chest is the level 6 BF key.
Act 4 BF key locations (Spoilers);
1 - starting level, in the Endurance test room in a cupboard
2 - lower maintenance tunnel level, close to the monster you need to kill for Nixfix
3 - summoning level, in one of the rune chambers you can teleport to
4 - philosophy level, concentration dungeon, in a locked wine chest (the key is hidden in a torch)
5 - philosophy level, consequences dungeon, under crates near exit teleporter
6 - end level, in a locked golden chest
