I would leave all four plys.
They say to remove 3/4 on old belt to get rid of the old spot the hooks into.
You have to also cut a small taper on the edge of the belt (as shown in illustration) on the trailing edge so belt doe not catch on something during use.
Then they say to just remove the ribs with a grinder so they don't interfere with the installation of the hooks.
I don't see anything about removing plys.
I spliced many belts and did just how I said. They all worked fine.