Allan,
For high availability on VMware you need minimum 2 hosts. So you will need to buy another Dell Poweredge R710.
Then you will have 2 possibilities but in all case upgrade your esxi to 5.x :
:2 hosts (R710) ,vCenter, VSA, VDP and HA.
1 - Install a vCenter VMware vCenter Server Virtualization & Server Management Software
2 - Implement VSA (Vsphere Storage Appliance) if you cannot buy a Storage Area :vSphere Storage Appliance - Shared Storage for Everyone. Here, VSA will share your local Storage and create Replica of your local datastore. (So if you have 2 hosts with 4TB each) , you will have 2 TB available.
3 - Implemement VDP (Vmware Data Protection) It is for backup your VMs. vSphere Data Protection Advanced: Backup and Recovery for vSphere environments
4 - Create a Cluster with HA (High avaibility) enable. if you lost a host, all your vm hosted on it , will be restart on the other.
In this case you will not loose your local storage (4TB).
2 hosts (R710), vCenter, Storage AREA, VDP and HA.
1 - Install a vCenter
2 - Install a storage area, using iSCSI, FC or NFS
3 - Implemement VDP (Vmware Data Protection) It is for backup your VMs. vSphere Data Protection Advanced: Backup and Recovery for vSphere environments
4 - Create a Cluster with HA (High avaibility) enable. if you lost a host, all your vm hosted on it , will be restart on the other.
I hope this will help you.
Regards,
Julien