A mono sub amp has only one channel, the second set of speaker terminals basically wires the subs in parallel for you.
The only way you can run a 4 ohm sub at 2 ohms is if you have a dual voice coil (dvc) sub. If your sub has two sets of speaker terminals then you have a dvc. If it has only one set, then you have only a single voice coil (svc).
If you have a dvc, then there are two ways to wire the sub so you can have a 2 ohm load at the amp. I'm assuming your sub box/enclosure only has one set of terminals, and if so wire the sub this way. Connect both positive (red) terminals to the positive terminal on the box, and both negative (black) terminals to the negative terminal on the box. then connect the enclosure to the amp using only one set of the terminals at the amp.
If you have two terminals on your enclosure you can wire it this way, or the way mentioned above. wire one voice coil to each terminal at the box, then connect each terminal to a set of the terminals at the amp, i.e. voice coil 1 to terminal 1 to amp output 1, voice coil 2 to terminal 2 to amp output 2.
those are the only ways to wire your sub at 2 ohms. If you have a svc, you're stuck at 4 ohms unless you add a second 4 ohm sub to wire into the amp.
Hope this helps