The interior angles of ∆AUL (or any triangle for that matter) sum to 180°, so
∠3 + 20° + 57° = 180°
===> ∠3 = 103°
Angles ∠3 and ∠4 occur on the same ray LP, so they are supplementary to one another. In other words,
∠3 + ∠4 = 180°
so that
∠4 = 180° - 103° = 77°
