If you put those two four ohm cabs in series, it'll add up to exactly the eight ohm the amp wants. IDK if there's anything you can buy to make that happen easily, but you could build a cable or a box with three jacks that would do it.
Take two speaker cables with good metal 1/4" plugs that you can easily take apart. Cut them both a foot or two from the end.
Strip the outer insulation on the two long ones and one of the short one back so you can get to the inner conductors and then strip those enough that they can be twisted together.
Now double check the wire colors by unscrewing each of the tree plugs and noting which wire goes to tip and which goes to sleeve. It is extremely important that you are sure about this and keep it straight! Do not assume that same colors mean same connection! Check for yourself.
Grab the two long cables and twist the tip wire from one to the sleeve wire from the other, and tape them off.
Now you have two wires left coming from those cables and they connect to the two wires on the shorter cable.
Tape those connections off.
To test it, use it as a guitar cable first. It will be noisier than a properly shielded guitar cable, but if you can hear the guitar about as loud as you'd expect, then you fucked up. If it does nothing but buzz like crazy, use a piece of wire or a screwdriver or something to short the free plug's tip to its own sleeve. If the guitar comes through now, you've done it correctly.
You probably should solder all those twisted wires at some point, but as long as it's a good solid physical connection and well insulated, it'll last around the house forever. I've used wire nuts in the past. Solder and shrink tubing would be best.
It actually doesn't matter which of these cables go into the head or which cab, you can plug them any way that works or makes sense to you.
