It seems in this case you're using the TMG as a firewall only.
Your configuration on your TMG should have a firewall rule to allow and external network to access and internal IP only on port 9005.
You will need to have network objects created, if you don't already, for each network.
For example:
External Net: Any/Any
InternalDVR: 192.168.1.5 for example
Firewall rule: External Net allow access to InternalDVR on TCP 9005.
What I find interesting is the order of your devices. Your TMG should have a listener, then the internal device should be the load balancer, and then the load balancer should front the four DVRs.
-If you find my post to be helpful, or the answer, please mark it appropriately. Thank you.
Chris Ream