K4 and Dengar crew are both very similar: They provide attack rerolls, cost 3 points and take a crew slot. K4 locks you on to one target. If you don't have that in arc he doesn't help at all. He also needs a green manouver to work, although the TL can carry over to the next turn. Dengar is a lot weaker against generics, but comparable against unique pilots. He would have been really bad in wave 3, but now he is great. Dengar leaves you entirely free to chose your target.
Which one would you prefer where and why?