Hi,
A few questions regards your setup:
i) are you using vCenter to manage your esx hosts?
ii) If so what licensing edition are you running on the esx hosts eg essential, standard , enterprise?
iii) If both hosts are running vCenter and have standard or greater edition it may be possible to use storage vmotion to move the VM
from your 4.1 to a 5.1 host, say may as there is a few requirements to be met, such as VM is running on storage available on both nodes, both VM's are
running on the same network
More approaches enclosed - is most likely you wont be able to storage vmotion so some downtime will be required unfortunately