Step 4 in the given proof is justified by the Alternate Exterior Angle Theorem.
The Alternate Exterior Angle Theorem states that when two parallel lines are cut by a transversal, the alternate exterior angles are congruent. In the given problem, a ll b and <1 = <4 are given. By definition, <2 and <3 are alternate exterior angles formed by a transversal intersecting two parallel lines a and b. Therefore, <2 and <3 are congruent by the Alternate Exterior Angle Theorem, which justifies Step 4 in the given proof
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