Scenario 4.
Destination nat: connection redirected from host A to host B in internal network.

Internal host 192.168.0.2 connects to 192.168.0.1, port 20026 Host 192.168.0.1, redirects connection towards port 22 of machine 192.168.0.3, where ssh server listens. 192.168.0.3 responds back to 192.168.0.1 and the last forwards the answer to 192.168.0.2, as you can see in the picture below.

In the picture above, you can see what happens in IPFIRE destination nat.