You need some kind of switch between the two HA pairs to make a 4 node. The NetApp CN1610 is the standard for a cluster that size.
The 2552 can take a total of 144 drives and up to 1468.8 TB of RAW capacity. per HA pair.
So you can have 144 drives in 6 (1 interna + 5 external). So.. if my quick math is correct, with 1.2TB drives you'd be maxed out at 5 (1 internal + 4 external).