vMotion esx 4.0 to ESXi 5.1

hi guys

I have a customer with a pretty old deployment.... vCenter 4.0 and ESX 4.0.

We are going to upgrade to 5.1.

I will be installing a vCenter 5.1 in new server which will have ESXi 5.1

so I will be adding this ESXi 5.1 and 3 ESX 4.0 servers to a new cluster so I am going to update one by one all servers

so my question is Can I vMotion VMs from ESX 4.0 to ESXi 5.1? any issue with vmware tools or virtual hardware?

thanks a lot