you will need to check to see if teaming NICs is supported on HYPER-V -
http://support.microsoft.com/kb/968703if you have a single additional 4 port NIC then if that card fails then you will lose network connectivity. if teaming is possible, try teaming one of the onboard NIC's with three on the add-on NIC for your VM's and the other onboard NIC with the fourth NIC from the add-on NIC for management.