Question: This might be a dumb question; I actually don't own SC2, so what experience I have is playing friends' accounts. But my friend has been playing on Diamond + Platinum, and so far he seems to be unopposed by 4 warp gate rush. Which is basically 4 warp gates + proxy pylon within 6 minutes, and sending in 3 zealots 3 stalkers.

Anyone who tries to tech up simply gets destroyed, because he has no units by the time the warp gets there. Is it because he hasn't been encountering anyone who is actually good?

And what are good counters to this, if I ever end up buying SC2 and playing against him?

I feel like this can't actually be a competitive build, because it doesn't seem to be the win-every-time strategy in professional games.

edit: I guess the more specific question I am asking is, "What are essential/crucial weaknesses in this build? (in general) In particular, the weaknesses that prevent it from being the end-all build"

Answer: 
Well, quite frankly it does dominate professional play kinda. Scouting the 4 gates is crucial when playing vs toss, and like Ivo said, the thing with the four gates is that you can continually warp from all 4 off one base. It uses all resources almost perfectly leaving no resources for anything else (unless you stop warping in of course) and makes for a steady stream of units at the front door of the opponent and when combined with chrono boost, it makes for unmatched unit production off one base. Its weakness? Hmmm, good question...in mirror match I know a toss can try 3 gate+ robo and forcefield their ramp giving them time to pump immortals. Terran can try same thing kinda, using extra bunkers to bide time to pump tanks. And zerg I suppose would need to throw down lots of extra spine crawlers to protect their xpo and just pump a mix of roaches and lings...basically if you see it coming you just do what you can to get ready to fight like...NOW!

And also I guess you can say that tactically, one of the weakness in the strategy would be the forward pylon...if for some reason you could take that out during the push it would help.

This is just my thoughts on the situation. I'm no master leaguer though, so I hope I'm not too far off from actually viable information here! I'm just tryin to help from the little that I know :-/
Good scouting against ANY rush is the exact way to counter it. What boggles the mind is that a 4 gate rush is still consistently winning in Gold and Platinum, for the simple fact that it is nearly completely counterable by good scouting, and any player worth their salt by that level of play sound be scouting more than say your average "medium" level AI.