Levels 1-3 are designed by Ariadne, "The Architect".
Level 4 is designed by Cobb, The Extractor, likely based on his memory of limbo (which, in turn, was designed by Mal "The Shade" and Cobb, based on their memory of reality).
Levels 1-3 are populated by Fischer's unconscious (with the occasional intrusion from Cobb's unconsious).
Level 4 is populated by Cobb's unconscious.
OR (alternative explanation of Level 4) Level 4 is unpopulated, except for Cobb's intrusions.