The only reason I can think of for "intermittent" 4wd operation would be electrical. It could be as simple as a transaxle switch adjustment.
As far as the actual 4wd operation, it works similar to on-road vehicles. Generally only one front wheel will drive along with one rear. If it were physical "4 wheel" drive, you would have a tough time turning and it would tear up the grass every time. Obviously, we can't have a mower damaging grass that it's supposed to mow!
The Toro Company