In Ji Xiaolan with Iron Teeth and Copper Teeth, why did Qianlong call himself Huang San Ye? Isn't he the fourth son of Yongzheng?

I've watched Ji Xiaolan with Iron Teeth and Copper Teeth four times. Ji Xiaolan was the oldest among the three tours in Ji Xiaolan, He Shen and Qianlong, so it was called Lao Ji, and the second tour in He Shen was called He Er, and Qianlong was named Huang San Ye. However, in history, these three people did not go on patrol together, so the title of Huang San Ye was also given to Qianlong by the novelist.