12" 4X SuperDrive not burning at 4X?!?!

I recently sold my old 1GHz 12" PowerBook with 2X SuperDrive and purchased the new 1.33GHz 12" PowerBook with 4X SuperDrive. I had some CompUSA DVD-R's that worked wonderfully with the 2X SuperDrive. Burning a DVD took just under 30 minutes. But now I burn those same DVD-R's on the 4X SuperDrive and in Toast it still says write speed as "Best" but it takes about 50 minutes for a 4.0GB DVD. I was wondering maybe I need to use some DVD-R's that are rated at 4X is that correct? The CompUSA brand DVD-R's do not have speed ratings but I assume they're 2X only. But I thought if I use 2X DVD-R's on a 4X Drive it would write at 2X?! Maybe it only writes at 1 I guess. If anyone can confirm this it would be great. At the mean time I will go buy some 4X DVD-R's and try them out. Thanks.