Add ESXi4 host to vCenter 5.5 failed, Host do not support VSAN

Hi all. I've build a test lab running with vCenter 5.5, and a Dell PE 1950 server running ESXi 4.  I am trying to add the ESXi 4 host to vCenter 5.5 (to a test cluster), however, the Add Host process failed with the error message, 'Host 'x.x.x.x' does not support VSAN".  Had any of you encounter this issue, and is there a way to by-pass this issue?  I do understand VSAN is a new feature on ESXi 5.1 and above, but I don't expect it to stopping me from adding ESXi 4 to the cluster.... Thank you.