Everything on 4:3s is 33% wider than on 16:9. The game goes for your aim. For the same amount of mouse movement, your crosshair will appear to move 33% faster (horizontally only) on 4:3s. When it comes to reaction time though, stretched doesn't make any difference at all. If an enemy appears at your crosshair (where you should be looking most of the time, especially while holding an angle) you'll be just as fast on 4:3s as 16:9. If an enemy suddenly appears on the side of you screen though, 16:9 (and 4:3) would be more advantageous. Reason for this is: if you play 4:3s and an enemy appears on the very left side of your screen, it'd be ALL THE WAY to the left of your monitor. If you were playing 16:9 and the enemy would appear at the same spot, the enemy wouldn't appear ALL THE WAY to the left of your monitor, instead the enemy would show up about 15-20% in from the left side of the monitor. This is closer to your 'focused area' than it'd be on 4:3s, meaning you can react to it faster. This is not the case for 4:3 though, obviously.