A 4" pair of scissors is not the most pressing issue to aircraft security. In light of what has happened, who here can honestly say that a plane full of passengers is going to let anything less than a full battalion of terrorists take it over? People are pouncing on other passengers that just ACT strange these days. I saw an article recently where a passenger with mental issues took his pants off during the flight and a group of other passengers attacked him and ended up handcuffing him to his seat.
Every flight isn't packed with heros, but given what people know now, think how they would react if 5 guys with 4" scissors got up and said they were taking control of the plane. I submit that it wouldn't last very long for those guys.
It is far more likely that explosives would come back "in style," either on a passenger or surreptitiously placed on board by a rogue ground crew member. Or, speaking of which, how about a gun or guns left on board by cleaning crew or other support staff? In that case no heros are going to re-take the plane.
I could list the doomsday scenarios for hours, but the point is that terrorists have brains, too. They're smart enough to know that a 4" pair of scissors or a screwdriver is going to be no match for 10, 50, or 100 angry passengers on a flight.