Calculating Frequency of Terms with Zipf's Law
The question gives you enough information to deduce $N$ and $s$.
You are given $f(1,s,N)$ and $f(2,s,N)$. Note that
$$
\frac{f(1,s,N)}{f(2,s,N)} = \frac{1/1^s}{1/2^s} = 2^s.
$$
Hence you can find $s$. In order to calculate $f(3,s,N),f(4,s,N)$ you don't really need to find $N$, since,
$$
\frac{f(1,s,N)}{f(3,s,N)} = 3^s, \quad
\frac{f(1,s,N)}{f(4,s,N)} = 4^s,
$$
using calculations similar to what we had before.

Notice that in order to solve this exercise, we had to know absolutely nothing about the Zipf law other than the formula. Stated differently, what you need in order to solve this exercise is not knowledge (apart from the formula for $f(k,s,N)$) but rather problem solving skills. One of the main goals of the undergraduate degree is to develop such skills.