Define dedicated server into a cluster, when one of them crashed

Dears Domino Administrator,

I am working with a Domino cluster with 4 servers, located into different sites:

S1 : NYC

S2 : Paris

S3 : London

S4 : Miami

How can I define those following rules:

If S1 is down, S4 will take the relay, same for S2, that S3 can take the relay? I can not find any connection document to do this.

Can someone help me?

W. STEVAN