I did a deck for a 24' round AG pool several years ago and I ran the deck boards underneath the 4" top rail.

It looked finished because the top rail hid any problems with the radius cuts on the end of the deck boards. If you leave the deck boards long and butt them up to the rail, you can transfer the radius to the boards and after the radius is cut, you can slide the board under the top rail to get your length.

Worked good for my application.