It says that's 4 units distant from the another vertex. So, if a square is formed by 4 equal sides and 4 right angles, then you just have to add 4 units on x or y.
-> (2, 1) -> + 4 units on X -> = (6, 1) or -> (2, 1) -> +4 units on Y -> = (2, 5)
(2, 5) isn't an answer so the unique possible answer is (6, 1)
