@Fanta: it could be that the Sony works better with the MAM-E than the NEC does. It could as well be that Sony thinks the results from a burn at 4x are good enough, while NEC may think that only results from 2x burns are acceptable.
MAM-E has discs that are made by Nashua, but also by lots of other brands. I've been using 4x DVD-R from MAM-E from a while (bought them @ CD Freaks shop) and they quite acceptable results, burned at 4x on my 1300a. I guess this just one of those brands that'll never tell you what you're getting... (unless you have a good writer or know for sure what you're buying, better stay away from it).