Download Nero CD-DVD Speed (or find it in the Nero Toolkit).
Pop in one of those Memorexes, and go to the "Disc Info" tab. You'll see the MID (also known as media code)...something like RITEK F1 or something as mentioned.
It's highly likely that if you're only offered 4x as a burn speed, that the new drives' firmwares don't support the mediacode, and therefore are burning the discs with a default (often 2.4x or 4x) burn strategy.
Added to that, Memorex is sub-par media and LG drives like to be fed good stuff.