You could use the separate left and right speaker outs - one for each speaker, but as Chris has already said, running them in parallel (two speakers from the same output) isn't safe to do.
You'll notice that your 4Ω cab is MUCH louder than the 8Ω, as it allows the full 250W to be delivered to the cab, but it's possible.
Another way to do it would be to run it in Bridged mode (8Ω minimum) with the cabs wired in series (giving you a 12Ω load), but you'd need a specialist cable to do that as it would need to run:
Amp hot ->8Ω speaker hot, 8Ω speaker ground -> 4Ω speaker hot, 4Ω speaker ground -> Amp ground.