"If I so wished, getting first place would be no problem at all, so what's the big deal about fourth place?"
Don't forget, Ling Xiao's Book of Seals still contains a Heavenly Lord sealed within.
No matter how powerful Ren Xing Tian was, he had yet to break through to the Heavenly Man Realm, so Ling Xiao's words were definitely not nonsensical ramblings.
However, the Book of Seals was a tool for preserving life in critical moments. If one relied on it too much, it wouldn't be beneficial for the enhancement of one's own strength.
That's why Ling Xiao doesn't use it lightly.
