OK, it's like this. Here's a simplified example... If you have two 8-ohm speakers, and plug one in each speaker jack, that's 4 ohms. If you have two 4-ohm speakers, that gives you 2 ohms. If you have 4 8-ohm speakers, that's 2 ohms (wired in parallel).
With solid state heads, you can run them at higher impedances, but not lower than rated. So if you ran that head at 4 ohms, you'd get less than the 200 watts, but it would work fine.
So for a 15" and 4x10's, each cabinet will have to be 4 ohms to get the full power. So figure on a 4 ohm 15", and each 10" speaker would have to be either 16 ohms, so all four in parallel would be 4 ohms, or 4 ohms each, with two pairs in series (8 ohms) and then the two pairs in parallel to give you 4 ohms.
2 ohm heads are pretty rare these days. And if you are buying speaker cabs already loaded, it might be hard to get them at 4 ohms.
Two 8 ohm cabs would work, but you'd get a bit less output.
Those are nice heads. I used to use one back in the 70's. You can hear them on the Who's Quadrophenia album.