Level Transit

I got a connecting flight, of the same carrier company (thai airways) after 3 hours from the time I arrive. Now, I have been told told that there is no need to have a transit visa as the flight carrier is same. I just like to know if its true.

If so, then, how can go directly from level 2 (arrivals) to level 4(departure).

Please reply if you really know or visited the airport yourself.
Please be sure of what you say.

Here is a link:

http://www.bkk-maps.com/suvarnabhumi-airport.htm

It says "In the case of transit passengers, they will have to go through the airside area to the departures service area in order to complete the departure procedures."

So, help me out with the right answer. Thanks.