Whenever I get asked about projector choices, I generally mention that the actual differences in picture quality are going to be relatively small when you compare the 6800 to the 4000 or 8500.
The qualities that I value in a projector are based partially on my viewing acuity (which is still 20/15 corrected), personal choices (love sharpness, hate projector noise), and my actual viewing room conditions (living room with some ambient light, 92" 1.0 gain Vutec screen, 16 feet throw distance).
Rather than mention what I don't like about the 4000 or 8500 (which I have not seen), I'll quickly describe what I do like about the 6800:
1) Fabulous combination of picture sharpness, color accuracy, and a non-digital looking image
2) Very bright in the "best" (cinema) mode. This allows me to use the lower lamp settings, reducing both noise and prolonging lamp life
3) Virtually dead quiet operation
4) True video blanking for optimal picture geometry/overscan setup. See previous postings in this thread for a more detailed description of this.
5) Focus, horizontal and vertical shift, and zoom are all motorized.
6) A Dynamic Iris that REALLY works. The best I've seen yet, hands down.
Since I use a 1:85 to 1 screen with no need for anamorphic settings, some of the "gee whiz" features of the 4000 are not that important to me.
Remember though, your mileage may vary!