To solve the equation \( \frac{4}{12} = \frac{x}{3} \), we can use cross-multiplication. This means we will multiply the numerator of one fraction by the denominator of the other fraction and set the two products equal to each other:
\[ 4 \cdot 3 = 12 \cdot x \]
Calculating the left side:
\[ 12 = 12x \]
Now, to solve for \( x \), divide both sides of the equation by 12:
\[ x = \frac{12}{12} = 1 \]
So, the missing value is:
\[ \boxed{1} \]
To verify, we can check if \( \frac{4}{12} \) simplifies to \( \frac{1}{3} \):
\[ \frac{4 \div 4}{12 \div 4} = \frac{1}{3}, \]
which confirms our answer is correct.