yes you can upgrade ESX 4.1 to 5.1, there is no issue in doing that.
You have to move all the VMs from one host to another hos and make the host free before upgrade.
for upgrade the hard disk in data store you have to do storage vmotion and move all the VMs to another storage node and make the Storage Node free.
you can do the Storage Maintenance and use that storage and datastore again.