To determine how many picnic tables are needed for 40 students, you divide the total number of students by the number of students each table can seat.
Each picnic table seats 4 people, so you can calculate the number of tables required as follows:
\[ \text{Number of tables} = \frac{\text{Number of students}}{\text{Number of seats per table}} = \frac{40}{4} = 10 \]
Thus, the students will need 10 picnic tables.