I wouldn't recommend mixing the different versions in the same cluster.
Although on the face of it you might not see any issues, there are a number of small changes made with HA and this could trip you up.
I would suggest either upgrade all the ESX 4.x or create 2 seperate clusters.
In terms of features not being available, I guess the main thing is support for DPM on a mixed cluster.
I do not think you can manage an ESX/ESXi 4.0 host with VC 2.5 - you would need to upgrade your vCenter 4.0 since vCenter 4.0 can manage a mixed environment and then that is only recommended as part of the path to upgrade to vSphere.
During the period where you are running a mixed environment on vCenter 4.0 you would not be able to upgrade your virtual machine hardware to verion 7 which includes the increase to the amount of memory, number of CPUs and NICs in your virtual machine to name a few. Also you would not be able to FT.
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