You have good modulation at the VCV, the problem has to be at the transmission modulator side. Try drawing on the line with your mityvac. It could be that there's a bad connector or the modulator is damaged.

Not to be pessimistic, but your 3/4 problem sounds like a bad clutch pack. I had a similar problem with my 190DT...I'd get well into third, and then it would shift to neutral. Required a teardown and rebuild. The friction plates and the clutch drum itself were worn out. But I could never get 4th to engage, so maybe you have a shot at it being a simpler problem. Getting into fourth places the greatest load on the hydraulic system, since both clutch packs have to be engaged. So there's a shot that it's related to insufficient working pressure. I assume your fluid level is correct?