Can 3 4/0 Alum. conductor triplex underground rated be in stalled in PVC conduit with a # 4 copper ground wire. If it meets 40 % fill single phase 120/240 volt. I'd say so want other views on this.?

I don't see a problem Ernie as I understand your discription. I don't know of anything in the code that would prohibit using a copper equipment grounding conductor with an aluminum feeder. and I'm assuming that the triplex consist of all insulated conductors. See Ya in Troy in December.

Same question. Is the neutral insulated?

Ground not a Problem , Neutrals insulated not a problem, I was curious about the Tri-Plex being in conduit underground?? Conductor fill for one thing I think would be different? Is tri-plex rated to be installed in conduit?? underground . I;ve also seen it done but wondered????

We did this a month ago. What is the insulation type ... USE? Other "wet" type?

What if its direct burial triplex??