Hi,

I would like to rent the cheapest apartments in BKK area. Say, 3-4k/month baht.

Not really BKK, but it's area that falls under the power of BKK immigration

Ratchada/Huay Kwang used to offer rooms at THB 2,500, on MRT etc. Take a look there.

What you are referring to in that bracket is not condos but rather thai style cheap apartments, lost around ...refer to the link below:

http://www.thaiapartment.com/

But please do your own due diligence and check out the areas first....a lot of undesirable locals and be careful...with the way that the locals are treating farangs in certain locales, it can be dangerous.

How old are you? Where are you from?

Lots of B2000-3000 rooms in the Lat Krabang-Hua Mark area not far from the Airport Rail Link. You won't find them online. Just go out to the area and hire a moto taxi and have him drive you around. Most such apartment have large signs attached with their phone numbers. You will find that many have no interest in renting to a "Farang"...too many problems. Best to have a Thai speaker along.