Insulation in the attic with ridge vent and eaves

4 inch thick insulation is thicker than a 2x4 (only 3.5" thick), which means you'll be compressing the insulation and decreasing the R value. Instead, I'd suggest laying the new insulation on top of the old (no need to throw out the old unless it's turned moldy), without a vapor barrier on the new (you only want one vapor barrier, against the ceiling), and going perpendicular (to the old to reduce the air gaps).

With 2x4 joists, I'd suggest that they are not load bearing and I'd recommend against putting any further load on top of them.

A layer of dust or dirt on top of your insualtion would be quite normal in a vented attic. If the dirt was inpregnated in the actual insulation, so that it was acting like a filter, then that would indicated air movement from one side of the insulation to the other. This would be seen most often around holes in the ceiling from light fixtures, plumbing/electrical or any areas open to air leakage from the interior to the attic. I think what you are seeing laying on top of your insulation is just plain old dust from outdoors entering through the venting. not a problem.