jmc

There are only a few ways to make one, two, and three digit numbers add up to $4$. The only one-digit number whose sum is $4$ is $4$ itself. Continuing with two-digit numbers, we note that the digits must be $4$ and $0$, $1$ and $3$, or $2$ and $2$. This means that $13$, $22$, $31$, and $40$ are the only two-digit numbers whose digits sum to 4. For the three-digit numbers, we organize the work in a table.

$$\begin{array}{ccc}PossibleDigits& PossibleNumbers& TotalPossibilities\\ 4,0,0& 400& 1\\ 3,1,0& 103,130,301,310& 4\\ 2,2,0& 202,220& 2\\ 2,1,1& 112,121,211& 3\end{array}$$Adding up the last column, we see that there are $10$ three-digits numbers whose digits add up to $4$. Adding those to the possible one-and-two digit numbers, we get $\boxed{15}$ pages in the textbook which have digits that add up to $4$.