Proper gauge soow wire?

I plan to upgrade my portable generator to a larger 40 amp max output unit.



When I originally ran the cable to my electrical panel I used 8/3 NM-B W/G wire to allow for a future upgrade.


My question is what gauge SOOW should I use for the 10 foot generator lead, 8/4 or 6/4? I'm a little confused as to why 8/3 NM-B is rated for 40 amps and 8/4 SOOW is rated for 30 amps?



Any information would be appreciated.


Thanks to all