GabesterOne: 4, because of updated graphics, improved interface and more plot twist in missions (fairfax residence as an example). Would love to replay it again, but sadly it is not digitally release yet. :(
I am a fan of Swat 3 more. Yes, Swat 4 had more modern graphics but the gameplay sucked in many ways for coop. However I can understand why people would prefer Swat 4 for adversarial games.
Swat 4 had fake mirrors and non realistic magical doors that always opened both ways. The game balance and fun was wrecked IMHO by some of the LTL weapons and the lack of ability for the host of the game to control which weapons were available.
I also remember having stutter issues with Swat 4 every time the enemy AI would fire. There would be a slight lag as soon as you got into a firefight. I tried the game on several different machines years ago and it would always happen.