I am going to modify my existing rolling rack which holds a Bass Raider to also hold a Hobie Pro Angler 12 below. My plan is to add 4" PVC pipe.

Is 4" PVC pipe the correct size?
Do I need two 10' pieces or will 8' pieces work?
Any suggestions before I get started?

I used 4" x 8' and it seems to work really well. I'll try and add some pics later today.

I worked on the rolling rack today and learned a few things:
1. Home Depot sells PVC pipe in 10' sections and not 8' sections.
2. 3" PVC pipe fits better than 4" PVC under the Pro Angler 12.
3. With 2 pieces of PVC pipe under the PA 12, there is a shorter distance between the 2 in the front than in the rear so screwing them down parallel isn't ideal.
4. The Hobie PA 12 doesn't fit on 3" PVC pipe in the bottom section of my rolling rack. The seat doesn't fit even when folded forward.

Rather than rebuild the rack with longer legs and engineer it, I cut a pool noodle in half and duct taped it to the lower 2x4s and shoved the kayak in there. It works for now and only took 5 minutes. Clearly, I was over thinking it.