"unending space"... you're funny.
The PS4100 has twelve drive slots and at this time you can order them with 3TB disks. This of course adds up to 36TB.
That's before implementation of course. With that large a capacity drive, running anything other than RAID10 will be suicide on rebuild times so... you lose TWO disks to hot spares, and two more to RAID. You lose some disk space to formatting.
So what's the math then?
5 x 2.72TB = 13.6TB that you can use to create LUNs and extend them to servers. Those servers will in turn still need to manage those volumes. If you ran RAID6 you'd get 21TB.
There will be at a minimum three IP addresses used by your array if you cable it properly. The group IP (used to discovery of LUNs) and each active interface (usually two on a PS4XXX series). When you add another shelf, it will consume two more IP addresses, but it should load balance the data across the two shelves as long as the disk types are the same and they are placed into the same storage pool.
So... two PS4100 shelves would net you 42TB of space you can allocate to servers via iSCSI. Note, there is a maximum of two 4XXX series devices per group, and groups are used to manage and pool disks together.