In that case you will need to connect 4 clients to one server port, the other 4 clients to the other port.
What I mean by that is that ib0 from the server and 4 clients should be in the same IP subnet. ib1 and the other clients should be in another IP subnet. Then on each client, iscsiadm -m discovery -p <ib0_IP or ib1_IP> should find the target, so the first 4 client will always go through ib0, the other 4 clients over ib1.
You don't need to do anything special in the IB switch. All you need is IPoIB on all NICs.
ib-bonding is for IPoIB only, I don't think RDMA packets will be sent over both ports. But apparently you can do that with RoCE ports in bonding mode, the only requirement is to use fail_over_mac in the bonding driver.
There should be more details in the OFED manual http://www.mellanox.com/related-docs/prod_software/Mellanox_OFED_Linux_User_Manual_v2.1-1.0.0.pdf