Fantastic.
I've been waiting for this forever - I nearly started crying when I heard it wasn't going up till the 21st (I'm a lurker on your site forums). So worth the wait, though :D
Anyway, the first 3 were great and this one is no exception. I really liked the ambients and the atmosphere and such. It took me forever to figure out what the circle-square-triangle meant in the tomb, I thought it was just about that one part in the ancient area, but I figured it out (circles have infinite sides, not 0! ;_;) and got all the secrets with no help.
Nice touch with the bonus material, by the way :D inspiration is all around you if you just look.
One minor bug- I was going through the game again, and moving really fast because I knew what to do- on the roof where you start out, if you click on the hammer too fast to break the padlock into the window (before that little speech bubble goes away, telling you to break it) the game gets stuck, the hammer won't break the padlock and you can't put it away. I don't know, maybe I just screwed something up cause when I started again, I had no problems.
Loved the ending, too. Truly bizarre.
Can't wait till the 5th one. There is gonna be a 5th, right? D: