Holy shit, it's out. GURPS Horror was the best GURPS Supplement of 3e, and possibly the best treatment of horror I've ever read (by the inestimable Kenneth Hite). This is the same work, updated to 4e, expanded upon to contain even more material, even more advice, even more monster templates, and a treatment of horrific powers on par with GURPS Powers.
If you're a GURPS Fan OR a horror fan, you owe it to yourself to check this out. Seriously.