This tutorial explains how you can set up MySQL master-master replication on four MySQL nodes (running on Debian Etch). The difference to a two node master-master replication is that if you have more than two nodes, the replication goes in a circle, i.e., with four nodes, the replication goes from node1 to node2, from node2 to node3, from node3 to node4, and from node4 to node1.