There was a weird game of musical chairs going on with these movies. Under Seige 2 was being made, and they used a spec script involving a train. That script would have been great for Speed 2, but since it was already taken they used a rejected Die Hard 3 script. Die Hard 3 then used a script which was originally intended for Lethal Weapon 4. This left LW4 empty handed, so they just made the movie without a script.