You first have to know what par is, or at least if you do it will help, and par is the predetermined number of strokes it takes to hit the ball in the hole. That means you have to swing 6 times to get it in the hole. When a golfer has a -4 that means they took 4 less shots than predetermined and a + 2 means 2 more shots than predetermined. So in the case, Vijay Singh's score of -4 would be less than Phil Mickelson's score of -2, -4 < -2. And in order of least to greatest it would be Vijay Singh, Phil Mickelson, Tiger Woods, Justin Leonard.
Btw I had to look up all the golf stuff. I know nothing about golf.