That happens from time to time. I do not think it is accurate though. It might be a communication thing.
Also may be some kind of refresh function of the Forum. I stay logged in all day, but I will let it sit idle for a few hours at a time. I think when you see the 4 Members online, you might be seeing the refresh accounting for "active" participants, or something to that effect.
I posted about his before, because it caught my attention once. The funnier thing, is when I see an enormous amount of people online, but no one is posting anything. Except the HOME section. That section NEVER stops.