I bought the old Painkiller games, the Black Edition and I started on the 2nd difficulty mode but it was too easy so I changed to Trauma, which is a lot more fun, but here's the problem; I completed Chapter 4's boss and I can clearly see there is a Chapter 5 but after I finished it the game just... Ends.. I really don't want to have to start over the entire game and re-collect all the tarot cards just so I can play Chapter 5 on Nightmare difficulty.

Isn't there any way to change the difficulty or to play Chapter 5 on Trauma? Is my game just bugged? Am I forced to restart the entire thing?
(Again, old Painkiller, not the new remake version)