4.

The time you entered a study. A lot of the books on the shelf are related to physics and mathematics.

Three books were floating in the air. They seemed to be rotating around each other as if they were stars and planets. But there were no patterns about their orbits. They were entirely random.

You decided to grab the books and skimmed through them. A few highlights caught your attention.
"We'll send only a brain."
"Weakness and ignorance are not barriers to survival, but arrogance is."
"He employed two basic methods: dual-layer metaphors and two-dimensional metaphors. The dual-layer metaphors in the stories did not directly point to the real meaning, but to something far simpler. The tenor of this first metaphor became the vehicle for a second metaphor, which pointed to the real intelligence. In the current example, the princess’s boat, the He’ershingenmosiken soap, and the Glutton’s Sea formed a metaphor for a paper boat driven by soap. The paper boat, in turn, pointed to curvature propulsion."

These books were hard science fiction. Some of the terminology used in the book is hard to understand without looking up the relevance topic. You now see the reason why there was a bookshelf of physics and mathematics book behind the desk. Maybe the exit was hidden inside the novels?

"刘慈欣," you noticed the author's name as you walked out. "So, these books are translated into English," you thought.

As always, another voice, "Maybe you should try the last letter of the author's first name"