PLEASE NOTE THE POST BY miles99999064 IS 8 YEARS OLD! You're not going to get a reply.
Assuming you mean pain immediately behind the kneecap, the most likely diagnosis is PFPS: patello-femoral pain syndrome, otherwise know as "runner's knee". Taking time off is best, and combined with the advice from JamesJohnson above, may be enough to get you to the starting line on time. Taping is unlikely to help. Being more careful with your footplant and stride (shorter stride, footplant under your hips) will help. A "cho-pat" strap (or similar) or a sleeve with a cutout for the kneecap may also help. Work on strengthening the quads in particular can help also.
I recently went through something similar (injury happened in a non-running incident). I took several weeks off and it wasn't worse, or better. So I started walking, trying to keep my strides short but frequent. After a week or so I was able to go to walk/run, then over a couple weeks run/walk. And the pain went away. Perhaps I was lucky but it may enable you to keep up some cardio. Or you could cycle.
At any rate what you want to do is resolve the pain in time for the marathon. Hopefully it will not take much time. Regardless, you will not lose too much conditioning in 4 weeks and should be able to complete the marathon, maybe with some downgrading of your goals.
And maybe I'll see you at MCM.
