In order to accomplish this, a major gradation into different levels was necessary in the fallen garments, and out of three they became four. Thus Malchut of Asiyah as it is now is only one part of the original Malchut of Asiyah, for the other parts rose higher and did not remain on the same low level. What this means is that since all the other levels were intent on separating themselves from the production of evil, they left only this level of Malchut of Asiyah to carry out this function, while all the other parts separated from it. They also all separated one from another (for example, part of Yetzirah rose to Beriyah, and so on), until Beriyah-Yetzirah-Asiyah turned into Atzilut-Beriyah-Yetzirah-Asiyah.
The Four Worlds and the Cascading of Evil
Kalach Pitchei Chokhmah 49:20