Since angles 3 and 4 form a linear pair, the sum of the measures of their angles must add up to 180.
We can define angle 3 as x, and angle 4 as (180-x).
x = 3 ( 180 - x ) + 4.
Solving for x, we get x=138. This is angle 3. Angle 4 is 180-138, so angle 4 is 42.