Few points here.
1. You can connect 5.x ESXi only to 5.x vCenter, you cannot add them to 4.x vCenter.
2. Whenever you perform upgrade, it is suggested to first upgrade the vCenter and then the ESXi hosts. This is so that after upgrading the vCenter, the 4.x ESX can still be connected to it because vCenter is backward compatible with ESXi hosts.
Now in case if you need to continue using your 4.x vCenter till the migration period is over, cant you put it in evaluation? Even after the evaluation period expires, you can still keep on using it as long as you are not adding any more ESXi hosts to it. Same logic applies to the ESX hosts too. Even if the evaluation period of ESX expires, you can still continue to use them as long as you are not powering on any VMs (existing powered on VMs will still work). Wouldn't that be ok for your migration period?