So while my main amp head is being repaired, i've managed to get my old one back.

However I want to be certain nothing explodes when I connect it all up.

It's a Kustom Quad Jr 100w head, here's the Manual

Now it's Solid State and says "Impedance: 4 ohms".
The cabinet I want to put it into is a Harley Benton 2x12, 120watts/8ohm, 2x 60watts/16ohm, mono/stereo (switchable)

Should I be okay running the 4ohm head into the 8ohm input, with the switch on mono?
Yes. On SS heads the rating is a minimum impedance, it's ok to use more but the power will be reduced.
Yep. With tubes or solid state you don't want to use lower than the impedance rating of the amp, both will handle higher impedance speakers with no problem, it will just make the amp work a little harder. Solid state will also handle lower impedance a lot better than tubes will, but is not highly recommended.

For your situation it will handle the 8 ohm load with no problems. I wouldn't use the 16 ohm option, but it would still work.
Quote by Paleo Pete
Solid state will also handle lower impedance a lot better than tubes will
Still you'll fry the output transistors if you try to use any more than the full power.

Yea TS, twice the minimum impedance will give you half the power (theoretically), and you may plug the thing into a load of whatever impedance as long as it's 4ohm or higher.
It will drop the power to about 60W, give or take (rule o' thumb is half plus 10%.) Still plenty loud enough for anything you'll need. As the others said, no damage will ensue.