In the third layer, there is a mountain and they ski down it and all, but after that, do they enter the fourth layer of dreaming or do they enter limbo? Is Cobb the only to enter and does he/do they do it by killing themselves or using their technology?
There are four layers, the last of which is called "limbo". Cobb and Ariadne enter the limbo layer using their technology (not by killing themselves) to get Fischer Jr. and Saitor out of it.
The link on CinamaBlend has an illustrated guide.
There are three levels plus limbo - the raw dream space.
- The Yusuf in Car level
- The Hotel Room level
- The Snow battle level where Fischer meets his dad and Saito dies.
- Limbo: Where Cob and Ariadne fight Mal, and Cobb stays back to find Saito.
There is a beautiful site dedicated for this one purpose: http://inception-explained.com/
It's unclear in the sense that, at the end, it is suggested that what viewers have interpreted as the "real" reality may also be a dream. [Recall that the penultimate action of Cobb is to spin the little top before going out to his kids, before he or we see if the top falls. A falling top indicates the "real" reality. We do see that top start to wobble, but Nolan cuts it off to make a philosophical point.]