Subtract 2 from 4, the number of sides in a quadrilateral, and multiply the result by 180 to find that all of the angles combined equal 360 degrees.
Add the measures of the other three angles in the quadrilateral. For example, if your other angle measures are 100 degrees, 130 degrees and 50 degrees, add 100 plus 130 plus 50 to get 280.
Subtract the result from 360 to find the fourth angle of the quadrilateral. In this example, subtract 280 from 360 to find that the fourth angle of the quadrilateral equals 80 degrees.