In any inequality, if the signs are same, we can square, cancel or cross multiply.
Here, both parts of the inequality are positive, we can square it and rephrase the question stem as
Is n + k > 4n ?
St (1):- k > 3n
Adding n on both sides n + k >3n (Sufficient)
n + k > 3n
This doesn’t tell us if n + k >4n (Insufficient)