The weapons used in these 4 hijackings would have gone through security anywhere in the world. Indeed as I recall from my UK training a knife up to 4 inches long is completely legal as long as you don't try to hide it.
The answer is definately to lock the door and armor the door in the short term. Now that everyone realizes what will happen if you let a hijacker in the cockpit, the thought of your cc being killed is extremely unpleasent, but less so than watching your aircraft fly into the tower of london...
In the longer term, access to the cockpit should ONLY be from outside the pressure vessel of the aircraft. Give the Cockpit its own lav, coffee maker and its own outside door and make it impossible to access the cabin. There is nothing to be gained from passenger access to the cabin.
Cheers
Wino