Why would you waste four jihadists, when one could accomplish the mission?
Ask the jihadist on 9-11 that question.
Seriously....in case one can not get on board???....in case it takes several people to bring the parts on board for the explosive device to pass security??? etc.
The 9-11 jihadists' goal was not merely to destroy the airplanes, but to take them over and crash them into infidel targets. For that you need enough muscle to get your at least semi-trained pilot into the cockpit in place of the crew. If the goal is just to blow up the airplane, one person should suffice. Actually, you don't even need one person aboard, unless you are into Allah and suicide and all that.
Seriously....in case one can not get on board???
Then you try again on a later flight with a different suicide guy.
....in case it takes several people to bring the parts on board for the explosive device to pass security??? etc.
Maybe. But have we actually seen any cases in which the explosive is manufactured on board from separate innocent parts? Or are they just theoretical threats DHS types dream up while sitting around conference tables?