A QNAP NAS with 4 drive bays can basically hold a maximum of 6 x 22 TB disks or even larger ones if firmware is required. If you set up the system with RAID5, this would result in a capacity of 5 x 22 TB, i.e. around 110 TB gross. Net, of course, it is less, probably just under 100 TB. But since a 22 TB disk costs almost 600 Swiss francs, this is not really a good idea. It would be better to buy a NAS with 8 drive bays, then you can also use a lot of storage, but you don't have to buy expensive disks.

My recommendation would be this one, it doesn't cost much more and you could also create a RAID5 with 8 x 16 TB, which also makes 112 TB of gross storage available. The bottom line is that the 8 drive NAS is not much more expensive, but you save a lot of money on the disks (16 TB costs about 400 vs about 600 for 22 TB):

QNAP TS-832PX-4G (0 TB)