you can use any 4" pad that has 4 inches of hook & loop (velcro)...
these look like they would work as the velcro extends all the way to the outer edge of these 4" pads, just keep in mind there is no margin of error since there is no recess on the edge...
the sister site has the older/discontinued hybid pads that were made to work with the 4" interchangeable flex backing plate. they are on BOGO right now...