[ExchangeList] Cluster question

Hello All,
 
If i have 12,000 mailboxes which is spread on 4 exchange 2003 mailbox servers, and i want to put them in a cluster how will the cluster be like? because exchange 2003 cannot take 12,000 mailboxes on a single machine (  i meant in a two node active passive cluster)
 
Do i need to create 2 or 3 pairs of 2 node cluster ( like the 1st 2 node cluster carry about 5000 mailboxes and the 2nd cluster carry 5000 mailboxes and the 3rd 2000)
 
OR
 
 can it be done on a 4 node A/P cluster.

 

Thanks

Raj07