Ch 4 says,
Leap years are years that are a multiple of 4 or 400, but not other multiples of100. For example, 2000 and 2016 are leap years, but 2100 is not.
This explanation is confusing and I don't know what the author meant.
So I searched the internet for why 2100 is not a leap year and found this explanation by NASA which explains it simply and clearly.
https://www.jpl.nasa.gov/edu/news/2020/2/21/doing-the-math-on-why-we-have-leap-day/
...years that are divisible by 100 don't have leap days unless they’re also divisible by 400. If you do the math, you'll see that the year 2000 was a leap year, but 2100, 2200 and 2300 will not be.