I would imagine that the 400 watts is power consumption, how much power the device will draw from the AC outlet, not it's output.
I'm not sure what a good rule of thumb would be. Speakers, amplifiers, and opinions vary greatly. Try a search, you'll get plenty of reading material.
I don't know a whole lot about the later transistor Sunns , but I would imagine that the output section is similar/identical to the other "Concert" series amps, and I believe the rating on those was 150 watts @ 4 ohms.
So, if this is correct, a 4 ohm cabinet should work fine; probably better than the 8 ohm JBL you mentioned.