In this opening, after 1. d2-d4 Ng8-f6 2. c2-c4 e7-e6 3. Nb1-c3 Bf8-b4, Why doesn't white play 4.Bd2?

 It has been played, but has never been popular. Basically speaking, the bishop is very passive on d2, you will have to move it again soon or later. Also the move does nothing to pursue white's strategic goal of pushing the e pawn to e4. Among others, black might play 4...d5, with a sort of QGD where white has played a suboptimal bishop move (black is even threatening 5...dxc4/6...Qxd4 due to the bishop interfering with the queen; of course the threat can be stopped but this just shows how Bd2 is doing white more harm than good), or just wait with 4...0-0. Then if 5.a3 black can play either 5...Be7 (the "loss of time" Bf8-b4-e7 is totally compensated by the fact that white will also need to move the bishop again, and the extra move white has gained in the process is the hardly exciting a2-a3 push. So the tempo count is actually favourable to black) or 5...Bxc3. After 6.Bxc3 Ne4 7.Qc2 f5 black can regain the bishop pair at will and has reached his strategical goal of conquering e4. He is at least equal.

4.Bd2 is hardly bad or dubious, but still unchallenging and needlessly passive. That said, as an occasional surprise weapon it has been played by very strong players (I think even Petrosian gave it a go).

Thanks guys

IM pfren and bresando have both given very good answers.

Why do you like the White pieces?  The advantage of the first move is very slight, amounting to roughly half a tempo.  It's not enough to win, but it gives you a half-step head start.  But in any race between the evenly matched, that half-step at the start probably won't be what wins the race, too much happens over the length of the course.

UNLESS the runner with the slight head start pushes hard to expand that lead into something more meaningful.  He is in control of the pace of the game; the opponent can't afford much dilly-dallying around as that lead lengthens.  But if the leader dallies himself, the other quickly catches up, and even passes him.

This is why we don't see 4 Bd2 often in the Nimzo.  White dallies, and he squanders his tiny lead very quickly by leaving Black many options and under no threat or pressure.  White has several options which do more to press his lead instead, and usually chooses one of them.

If you prefer to keep your pawn structure at the cost of tempo, I'd reccomend Qc2.  It's more active and strenghtens the e4 push.

Well, I dont like to play aganist nimzo so I play 3. Nf3 instead.

 I heard alot of players try to avoid the nimzo indian as white..can anyone explain to me why alot of players tend to avoid it?

yeah but what are the challenges does white face when he has to go up against the nimzo indian? I understand alot of people don't like playing against it...but I want a lil bit more details

Nimzo is the second most succesful opening for black just after sicilian. So, it's the top choice for playing against d4

It's a very strong and flexible defence; black has not committed his central pawns and so can play according to several different strategies. This means that it's a lot of work to prepare something against it, and white can't just apply a one-size-fits-all plan. He has to know how to play several different pawn structures.

When playing against the nimzo white has a few reasonable options. When I put together my QGD repertoire I was playing the Qc2 lines. White generally lags behind pretty far in development and must face down a little initiative from black. Personally I hate defending as white, and if you're not extremely accurate your bishop pair goes away and you're a little worse.

Another annoyance is that many of the more aggressive lines for white both in Qc2 and in the recently more fashionable e3 lines leave you with a long term structural weakness that black is already poised to exploit (the c4 pawn is weak). White then must make something happen very fast, and energetic play like that is not always so easy.

Because it is very complex, Black has several good options at almost every turn, and it requires very precise play to squeeze any significant advantage for White out of any of the main approaches.  Add to that the transpositions in the first few moves, and for some White players there is just too much effort for too little reward. 

This really accounts for the popularity of the Catalan at high levels, IMO, because the Nimzo doesn't promise enough for White.  And as little as the Catalan offers for White, that says a lot.