Your practice needs to fit your usage needs. I would not want to add more categories to enter data into or retrieve it from than I really needed (four is too much thinking for me) but maybe this works for you. It is not the pure GTD system because you have blurred context with the reason the loop is open but that might be a best practice for you. Something to consider if it applies to you, as it does to me, is that while you are "waiting for" a response you generate another agenda item for that person, what happens if you are waiting for a reposne to a paper item and then you are waiting for an e-mail from the same person. When you finally catch up with him or her, if you just go to agenda you may neglect waiting for items or vice versa. Maybe that never happens to you because your world works differently from mine. I also have this terrible fear that I end up punishing people when they finally get back to me because I need another response from them, so gradually they may avoid me for as long as possible and then the list grows longer.