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: 
The pros have 4gate pretty much figured out. 

The tell is, if you scout at the ~14 food mark:

* one gas
* one gateway
* one cybernetics core and
* lots of energy in the Nexus.
* (the last three gateways will most often be placed down all at once)

As holycow has mentioned, each race has its own reactions to it, but I'd list different reactions: 

* Zerg has a queen that tanks *a lot* of damage, one or two spine crawlers deal a lot of damage, and speed-upgraded zerglings make everything just fall apart for the Protoss.
* Terran only needs to make two bunkers filled with a mix of marines and marauders on top of your ramp, and Protoss simply can't make it up the top. The concussive shell upgrade makes the defense absolute.
* Protoss' counter, unfortunately, seems to be a defensive 4-gate. This is very tricky and micromanagement heavy, but it's doable. Another defense would be a 3-gate robo with immortals tearing through stalkers, which will also make you ahead in tech. But this is very hard to do, and very prone to mistakes.

The point is: 4-gate is a very formulaic attack - you can analyze one 4-gate, and you've analyzed them all. You can practice against it, and once you figure it out, there's practically no 4 gate that can kill you anymore.

And, this is probably going to deaf ears, but while 4-gating might give you oodles and oodles of wins, they're empty wins. If you get into diamond simply by 4-gating, you *will* hit a brick wall: 4-gate is all you have, and it won't work anymore. So you'll start to lose *lots* of games, while you try to actually learn the game.

*BTW: the next patch (1.3.3) will make 4-gate a very different beast, since the build times are juggled around a ton. The lesson in this: don't rely on a single all-in build, if it gets changed, you're screwed.*
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 :-/