To find the value of \( x \) for the two adjacent angles that sum up to \( 90^\circ \), we can set up the equation:
\[ (x + 4) + (3x + 2) = 90 \]
Now, combine like terms:
\[ x + 4 + 3x + 2 = 90 \] \[ 4x + 6 = 90 \]
Next, subtract 6 from both sides:
\[ 4x = 84 \]
Now, divide both sides by 4:
\[ x = 21 \]
Thus, the value of \( x \) is \( \boxed{21} \).