IT IS Possible to Create Cluster Between Physical ESXI V.5.1 AND Virtual ESXI v.4.1.0 ?

Hi,
i want to create Cluster.

i have 1 Physical sever with ESXI 5.1 And Virtual ESXI 4.1.
IS IT  Possible ? IF THE ANSWER IS NO SO
What are my Options ?

1. 1.upgrade
   my virtual esxi 4.1 to 5.1 ?
2. 2.install
   new virtual esxi with 5.1 ?

Thanks,Guy.