If you are wondering if you can run your vSphere 5 lab nested on ESXi 4.1, the answer is yes.
I used Eric Gray’s (@eric_gray) procedure VMware ESX 4 can even virtualize itself to create the VMs. For the guest OS type I tried Red Hat Enterprise Linux 5 (64-bit) and Red Hat Enterprise Linux 6 (64-bit) and both worked without a glitch.
Here they are running on top of my whitebox, which is running ESXi 4.1 Update 1, the left one (esxi5) is created as RHEL6 and the right one (esxi5-02) RHEL5.
I added also the monitor_control.restrict_backdoor option but have not try yet to run nested VMs. I’ll do later and will update the post with the results.