Depends on year here in the US, I’ve owned an older FL412 that had 8 ohm speakers and was 8 ohm mono/ 16 ohm stereo. If your cab says 8 ohm mono it’s running all 4 speakers when connected to that jack.
Newer cabs here in the US are opposite, 16 ohm speakers so 16 ohm mono, 8 ohm stereo (2 speakers).
Hope that helps?