Find the Square Perimeter Function: Side Length 'a' Expression

To find the perimeter of the square with side length $a$, we use the standard formula for the perimeter of a square:

$P = 4 \times (\text{side length}) = 4a$

This straightforward multiplication tells us that the perimeter is four times the side length because a square has four equal sides.

Therefore, the function that expresses the perimeter of the square is:

$y = 4a$

Among the given choices, the correct expression corresponding to this calculation is: $y = 4a$.

Thus, the solution to this problem is $y = 4a$.