I am gonna try to implement this myself since Vadim S. demo is crashing for me at certain camera position/angles, also his code is hard to follow and unreadable so i cannot fix it
I think the problem is in the clipping algorithm, sometimes, when triangles intersect the near plane (an assert in there is the cause of the crash)... but yeah, his code is very hard to follow.
i could test just 4 pixels/corners of occludees axis aligned 2D rectangle?
I'm not sure that would work -- e.g if the occludee is just beyond a window, but is larger than the window. It's four corners will be occluded by the wall, but it's centre will be visible through the window.
If you're rasterizing a traditional depth buffer instead of one of these 1-bit occlusion buffers, then you can create a hierarchical Z-buffer from it, which does allow you to test any occluder with just 4 samples. This is a very old technique (1993?), but was used very recently in a splinter cell game.