Angles three and four are a linear pair, meaning that they they form a line. When two angles make a line, the measures add up to 180°. Adding the two values together, we see that 3x+27x=180. Simplifying and solving for x, we get 30x=180 ⇒ x=6.
The question, however, is asking for the measure of angle 4. Thus, we plug x=6 into 7x to get 6(7)=42.
Hope this helped!
