If angle 1 is congruent to angle 4, angle 1 must also be congruent to angle 3 by the transitive property. This is because is angle 3 and angle 4 are vertical angles, and they must be congruent. If angle 1 is congruent to angle 3, then the two lines in the middle must also be parallel due to the converse of the corresponding angles theorem. Because the two lines in the middle are parallel, angle 3 must be congruent to angle 2 by the alternate interior angles theorem.
