Dan's answer is of course correct, although the appropriate terminology for an dual-controller system is an HA pair or HA system; cluster is the term for multi-node GX or Data ONTAP 8 clustered systems.
The reason the DS4243 shelf cannot be used in either stretch or fabric MetroCluster configurations has to do with SAS specification limits on cable lengths: the current maximum supported passive SAS cable is 5m. SAS use copper cables; there is no physical interface for fibre and switches.
Skip Shapiro