To find the equivalent ratio of \( 12:9 \) in the form \( 4:x \), we can set up a proportion:
\[ \frac{12}{9} = \frac{4}{x} \]
Now we can cross-multiply:
\[ 12x = 4 \times 9 \]
Calculating the right side gives:
\[ 12x = 36 \]
Next, we solve for \( x \) by dividing both sides by 12:
\[ x = \frac{36}{12} = 3 \]
Now we can express the ratio \( 4:x \) as \( 4:3 \).
Thus, the equivalent ratio is:
\[ \boxed{4:3} \]