While Cobb was in “Limbo” trying to find the Asian dude, he was so focused on getting back to reality and seeing his children again, that he was sub-consciencely being the Architect for the fourth layer of dreams, being the reality where he wakes up on the plane and goes home to see his kids. Note: It never showed the Asian guy or Cobbs take themselves out of Limbo, it just showed the Asian guy pick up the gun. Now, like I said this is my take. The movie is obviously open toquestioning.