So the least common multiple of 4 and 10 is: 2×2×5=20.

In other words, they will both be in the library in 20 days.

According to the time when they went to the library for the first time, it can be inferred that the next time they went to the library is April 25th.

A: They met in the library on April 5th, so the next time they all go to the library is April 25th.