If I were you, I upgrade my 4.0(5) to a newer version.. Failover works much better.
The Failover process will actually grab the license file from the primary.... so you will need to install, reinstall the original file. When you configure the failover server, just install like the primary, but with no license file. Run failover wizard on the primary, then on the secondary and test. It can be an automatic process, or a manual failover. The Failover Monitor works well and is easy to use.
If your primary license with the NIC card has Failover license and failover licese for subscribers, you should be all set
hope this helps