These are the best 4X DVD-R I've used so far. Had only 1 coaster out of 100 + burns and that was because I tried to listen to mp3 and browse the web while burning at 4X (dumb).
I've tried all kinds of 4X media on my DRU500A and got mixed results. Memorex 4X burns at an abysmal 2X. If I burn at 4X, they mess up 50% of the times. Generic 4X discs (don't know what brand, bought 2 different packages) had about the same result as the Memorex discs (they are CMC Magnetic). No problem burning any 2.4X DVD+Rs though. And those cheap 2X Verbatim discs were pretty good.
Also, this is a real deal (25-pack 4x DVD-R (Ritek G04) @ Pc Club for $19.99) if you can pick them up at a local store. Otherwise, buy
these for about a buck a disc with shipping options starting at $5.