Because those 4 pax are actually staff traveling, on staff tickets, and if the flight is fully loaded, then the said 4 can be seated in the spare jump seats (2 at the rear of the a/c, 2 in the flight deck)
your guesses are wrong
Ryanair staff tickets are stand by tickets, whatever the number of overbookings. If the flight has no overbooking and 189 paid PAX, there could be anyway 20 staff tickets that will wait at the boarding gate
I said that few posts again, Ryanair lie again to its passengers, by saying that officially they don't overbook, whereas indeed, they overbook many flights.
Usual Ryanair bull****, but who's surprised???