I have this argument all the time with server administrators. A heartbeat should be on a cross-over cable between the two servers, not over the LAN. Oh well, so be it. So it's on the LAN...right, what to do.
One option is to use another network card on each server if it's available.
Or instead of bonding the 4 NICs, what about using 3? Is it really necessary to use all 4? Is there really that much traffic?
Monitor the server / switch porta for traffic usage. If the are ethernet ports have you thought about upgrading to fiber? And leave the copper for heartbeat?
I eventually got them to see the error of their ways and now they put crossovers. You don't won't servers switching node for a switch failure if you have desgined it properly.
For instance you should have two switches. Eacher server with one or more interfaces bonded to each switch and a crossover inbetween. There you have a redundant configuration for network failure and for server failure.