Rask!... you forgot this?

I'm sorry to hear you no longer have 1200 Watts at your command.

8 Ohms parallel with 4 Ohms comes to 2.6 Ohms. If your Ampeg is transistor, it should be OK. If it's a tube amp, you should try to get a closer match to the output transformer tap selection.

The 8 Ohm Traynor 2x15 is probably loaded with a pair of 16 Ohm speakers wired in parallel. Back in the day, nobody was stuffing 15" 4 Ohm speakers into their cars, so they weren't very common. The 4 Ohm cab you already have, probably contains a pair of 8 Ohm speakers in parallel.

I can't think of any tidy way to combine these to get a better impedance match for a tube amp output transformer. Since both cabs have two 15" speakers, I suggest doing an A-B comparison, and keeping the cab you prefer. Then get another cab, like a 4x10, to match up with that 2x15 cab.

The Ampeg Forum may be helpful in choosing a good combination. The consensus seems to be that two cabs, with different speaker sizes, gives a better balance of 'punch' and 'thump'.