What angle is complementary to < 4

The answer is:  " 3 " ;  that is:  " ∠ 3 " .

From the image attached,  " ∠ 3 " is complementary to " ∠4 " ; since "complementary angles", by definition, add up to "90° " (i.e. "completementary angles form a "right angle" ;  and "∠ 3 " and  "∠ 4 " , together" form a "right angle" (i.e. form a "90° angle").

As such:  " ∠ 3 "  is complementary to "∠ 4 " .