All four planes shot down? Huge upset and heads are going to roll. Nobody would know they were going to be used as missiles, everyone would assume they were just being hijacked for hostages. If George Bush stays in office he wouldn't get re-elected.
Also, if the first two planes are shot down over NYC you would see a lot of damage to the city.
The death toll if the two aircraft over NYC had been shot down would almost certainly have been far higher than was actually the case.
More importantly, the ONLY way the first two aircraft fall to AAM is if the Air Force is laying in wait. There simply is no other way to flush the fighters, acquire the aircraft and shoot them down. From the time Flight 11 stopped routine communication (08:14) to the time that the second aircraft, Flight 175 hit the second tower (09:03) you only have 49 minutes. Even if you have fighters on a Plus 15 alert you will never get them up and locked on in time, not in peacetime.
As was the people at Otis ANG base (in Massachusetts) did one hell of a job, they put two Eagles up 20 minutes from the initial call received at 08:34 (wheels up at 08:54) and had them off NYC by 09:09. Flight 11 hit WTC #1 at 08:48 or 14 minutes after the initial call to the ANG. At full throttle (no afterburner, or around 700 MPH, 0.95 MACH) the flight time to South Manhattan, which is 180 miles from OTIS ANG, is around 15 minutes. Today, the closest base is in Burlington VT, or about 25 minutes away.
If the first two aircraft had been shot down the Truthers (crazy bastards one & all) would be right. The Government (or at least the military) knew about the hijacking well in advance. Only possible way to have it happen.