Green stickers as seen in car windscreens

Wondering if anyone can help me with this one - its been puzzling me for a while. A large number of cars (I would estimate 30-40%) that I see have a (usually) green sticker in the windscreen with the number 4 and the registration number of the car written on. What are they? They aren't residents parking permits because there are cars in my street with both the paper residents permit that I have and these stickers, and I thought all of the other "official" stuff was done by stickers on the reg plates?
They seem to be in cars from all over the place - so not just a local phenomenon. Anybody know the answer?

Emissions stickers? Particulate matter perhaps: Fine dust particles, traffic pollution, and health

They are emission stickers. They range from 1 to 4, 1 being red and 4 being green. I heard that sometime in the future cars with levels 1 and 2 won't be allowed in certain cities.

I see...I've looked at the link to the earlier thread & several of the links from that - but I still can't work out (could well be my shoddy German) how do I find out where these zones are, so I can work out whether I need one or not, and if I do need one where do I go to get it - the same place I got my TUV?

I'm pretty sure that Berlin is starting to set restrictions as of 2008. I think you get them from the same place as your TUV.

Everything inside the S-bahnring will be an Umweltzone starting Jan. 1st 2008. For Munich, it will be inside the Mittlerring and start Oct. 1st 2008 at the earliest.

The stickers cost between 5 and 10 euros and pretty much every place that has abgas capability can do it. However, this does not mean that your car will pass and there is the likelihood that you will need a particle filter installed.

So, who needs them? I switch cars every 6 months and there is no way I'm going to pay every six months to get a new sticker so that I can drive into the city for my German class. Are new cars exempt?

The last time we had one of these schemes, back when it was all fields here, was when Katalysors were just coming in and you got one of the then orange stickers with the new car if it conformed with the emissions requirements. This was more about combatting smog , but amounts to the same thing. I would imagine that the new scheme will run the same way. If not then you won't be able to use your car in the city centre, if there is no way you're going to pay, then there is no way you will be able to use your car. Choice is yours.

Maybe the BMWs will come with them already on the car (after all, I don't own it or even lease it from them, I rent it from them - it is registered and insured under BMW, not me). Hmmm. I will have to find out.