The Preferences>Draw>Adots slider determines the number of pixols in size at which a model will switch to displaying as dots while you’re transforming it on the canvas. The higher the setting, the larger you can draw your model before it will temporarily switch to the dots view. But as you’ve already seen, performance slows even when it switches to dots. Without that switch, it’ll be even slower.
Document size is NOT the key to successful high resolution texturing. Well, not in the way you are thinking, anyway.
If you’re using Projection Master, you want to get a one-to-one ratio between the pixols displayed on the canvas and the pixels being written to the texture. To achieve this, you’ll NEVER use a 4096 canvas. Far from it! I cover this subject in detail in this tutorial: http://www.zbrushcentral.com/zbc/showthread.php?t=27680
Want a simpler solution? Use PolyPaint! Simply subdivide your model to have approximately as many points as your texture will have useable pixels. As a general rule, a 1024 texture needs 1 million points in the model. A 2048 texture needs about 3-4 million. A 4096 texture needs about 12 million. (Most textures have about 25% dead space.) After dividing the model to the necessary number of points, paint on it directly in the viewport. You can even use ZAppLink 3 to send the model to Photoshop for some stuff. When done, convert the polypaint to a texture. And the beauty of this method is that you never need to worry about canvas size at all. Just work at the canvas size that fills your screen, which will give fast and smooth performance. You can find more info on PolyPaint in the online documentation. See my signature.
