What pair of letters comes next in the series BF, DH, GK, KO?

In every pair the second letter is four places past the first: B 2 and F 6, D 4 and H 8, G 7 and K 11, K 11 and O 15. The first letters themselves are 2, 4, 7, 11 — a step that grows by one each time, so the next first letter is 11 + 5 = 16, which is P, and the second is 16 + 4 = 20, which is T. Repeating the step of four for the first letter gives OS; taking the second letter three places on instead of four gives PS; letting the step grow to six gives QU.