I replicate my VM's to ESX 5 from my ESX 4 hosts. Why I can't fail back to my orginal VM's after failover.

When replication occurs, new VM is created called <VMname>_replica this VM is hardware version 8, the source's VM doesn't matter.
Once the failover happens the VM being hardware version 8 can't be replicated back to ESX4 because it does not recognize hardware level 8.
Please upgrade all your source host servers to match ESX version of your target hosts.