I think you need to clarify the question!? As mentioned by the other posters before, you won't be able to run multiple vCenter Server versions concurrently on the same Windows system. However, if you are asking whether it is possible to manage ESXi4 as well as ESXi 5 hosts using a single vCenter Server 5 instance, than the answer is yes, this will work. In any case take a look at the VMware Product Interoperability Matrixes to ensure your specific versions/builds are supported to run together.
André