After Saito dies on the third dream level, he enters limbo. Ariadne then convinces Cobb that it is necessary for them to enter limbo to go after Saito, in order to pull him out. They do so by hooking themselves up to a dream-machine on the third level, within the snow fortress, and use this to enter limbo.
How are they able to enter limbo in this way? It had been previously explained that to die in a sedative-assisted dream would cause one's subconscious to enter limbo (and this is in fact what happened to Saito). But wouldn't using a dream machine on the third-level just cause Cobb and Ariadne to enter a fourth level? Why do they end up in limbo via this mechanism?