The blurb on the product page ends by saying "All this power guarantees epic games in no time . . . starting with the new adventure that comes in the box!" so my money is on the fifth book being a 64 page dungeon module.The 'something else' is probably the core rulebook.
Also, GURPS Mars Attacks is at print and set for release late this year.Something that got a bit lost in all this: GURPS Discworld is finally coming, as well! AND it's getting a print release. ... Hot Damn. GURPS Dungeon Fantasy + GURPS Discworld! I know what I'll be planning soon.