I happen to use the NC3 also. I had been playing an NC4 for quite some time, and then a friend let me fool around with his NC3 and I fell in love with it. I think that your problem sounds more like something that you're doing with your embouchure as you seem to already know. I think it would be wise to try an NC4 and see how you like the piece, although I don't know if switching mouthpieces would necessarily correct that problem. While the NC4 is certainly more common as you said, I think that it is an individual choice to make, and that trying them side by side would be a great experiment for you to do. Not to start an argument or anything, but I don't think that what mouthpiece you use is a good measuring tool for evaluating the maturity of your embouchure...everyone's different!