RAC design question

Hi

I have a RAC design question. We have two IBM Power P750 machines which one is split into two partitions. Interconnection is 10G ethernet. About RAC design, We have two plans. One is create two 2-node RACs, the other is create a 4-node RAC. Even if we used 4-node rac, we will still deploy the DB(SAP DB) in only two of them ,and other nodes will run other DB.

Many experienced guys advise we to create  two 2-node RACs. They say 4-node rac performance will not be very good using traditional SAN and ethernet. And also,  the difficulty of implementing 4-node is more complex.