Now I have the decorative raught iron collumes, will the 4" pvc/8' hold the wight of these old rusted collumes?

I lost my text book for doing the stress analysis.

I found this link which looks promising: pvc pipe compression strength

They suggest concrete with rebar in the PVC. Not sure how you might accomplish this.

I might also suggest polishing off the markings.

There has to be decorative column coves. I see them all the time in shopping centers.

If you're going to use 4" PVC for supports I would use schedule #80 and fill them with cement. Good luck, Tom