Can anyone point me in the right direction for replication compatibility between different model EqualLogics?

Im wanting to add a replication partner to my current SAN infrastructure as an extra level of backup

We have x2 SAN's which are not in the same group, these are:

PS4100

PS6100

Im looking to replicate data off both of them, or at least the PS4100. Can this even be done and if so would I need the same model SAN to act as the replica partner?

We are using iSCSI

Thanks