To find the value of $4y$ when $y = 2$, we perform a simple substitution into the expression. The expression $4y$ represents $4$ multiplied by $y$. Hence, when $y$ is equal to $2$, we substitute $2$ for $y$ in the expression.

This substitution leads us to calculate $4 \times 2$, which results in $8$.

In mathematical terms, substitution is a clear and straightforward process where a variable is replaced with its specified value. In our case, the variable $y$ is replaced with $2$. After this substitution, we proceed with the multiplication as follows: multiplying $4$ by $2$ yields $8$.

This type of problem is frequently encountered in algebra, where evaluating expressions by substituting variables with specific values is common. Mastering this skill is essential for tackling more complex equations and for working with functions. It is a foundational technique that is applicable across various fields of mathematics and science.