The number is smaller, fewer or less than?

I always make confusion about the correct usage of the comparative for "irregular" adjectives (I don’t know if this is the correct term).

Recently I had to write "the number of X is ____ ?", while I wanted to say that the quantity of X this year is less than the quantity of X in the previous year.

So, which of the following would be correct? Are there forms that are grammatically correct but not used?

1. The number of students this year is fewer than the number of students last year
2. The number of students this year is lesser than the number of students last year
3. The number of students this year is less than the number of students last year
4. The number of students this year is smaller than the number of students last year

I know that fewer is used for countables (e.g. "I have fewer children than my friend", or "I have fewer cars than a billionaire would have"), while less for uncountable (e.g. "I have less money than my friend"). Am I correct on this?

So, since "the number" is countable, I would opt for 1, but I am not sure.