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