The VM import and export will always be an option for you but depending on your storage configuration in your environment I believe you can make it more easier for you. If there is a possibility of your 4.1 host and 5 farm can use this as common storage , then you should be adding your 4.1 hosts to 5 vCenter and do VM hardware update>VMtools update>Storage vMotion and ETC.
1- Create a new cluster in 5.1
2- Leave HA and DRS off
3- Add the 4 ESXI host to new cluster
4- SVmotion and Vmotion the host to desired cluster ( You may have uncheck Nic or remove from VM if you don't have same VLANS available in the desired cluster but I'd prefer you create a new VLAN even if it is not trunked to physical NIC of ESX hosts)
5- update the VM hardware and tools
Note- You need to review your ESXi 4 license or you can utilize your existing V5 licenses and speak to to VMware I believe it should be backward compatible.