It's almost definitely the Moserbaer media then. Trying it on your friend's BenQ might not really be of much use. See, it is possible that Moserbaer media just does not burn properly with a 4163B but might work with a BenQ. If that is what happens, you can just sell him the rest of your Moserbaer discs! On the other hand, if they don't work well with his BenQ, then it proves they really are poor quality media.
As for writing at 4x, it should not matter with the 4163B. For one thing a disc rated at 8x is made using dye that works best burned at 8x. Also the 4163B seems to do good quality burns with decent media at the rated speed the media is supposed to burn at. So your Sony 8x DVD-R should burn with decent quality at 8x on your 4163B. No need to burn at 4x.
Oh btw, I've tried Sony 8x DVD+R media (media id SONY-D11-00) and it seems to burn fairly decently on my 4163B. So if you can find that, it should work ok with your 4163B as well.