Upgrading the Readynas Pro 4 (RNDP4000/4220) with 4 x 8TB disks

You should be able to use the full capacity of 4x8TB drives (after redandancy and overhead) with OS4.2.x, but you really should at least try OS 6.6.x since you are going to have to factory reset, anyway.  You can go back to 4.2.x if you don't like it (with another factory reset), but I think most who have made the switch have grown to like 6.6.x over 4.2.x.  There are still a few things 4.2.x had that 6.6.x doesn't, or at least are very different, but there is a lot more that 6.6.x has that 4.2.x lacks.

With a room temperature of around 72F and 4 2TB Seagate Enterprise drives (NS suffix -- these guys produce a lot of heat), my RNDU4000 fan is running at 1771RPM, whihc should be similar to an RNDP4000.  But it's not very noisy.  If yours makes a lot of noise at that RPM, you may have a failing fan.