With regards to the rear access ports, I would recommend staying with the 4" port since the 5" port's hole thru the top of the hull will put you too close to the ~3/4" diameter bead of hard dried resin (that oozed out, on the inside, between the sides and top of the hull when it was manufactured). This bead will also get in the way of putting all the bolts thru for the 5" access hatch.
I have big hands, so I decided to cut the holes for the 4" ports, do all my work on thru bolting the gudgeons before I installed the 4" plastic ports. The hole that you will cut thru the top of the hull will be ~ 4½" in diameter vs ~ 3½" for the inside opening of the 4" port.
Bill Mattson wrote a good article in On The Wire entitled: Do-It-Yourself: Installing Deck Portshttp://www.thebeachcats.com/news/29/ins ... eck-ports/