Recording on 8x with the matsushita drive-> 13:44. => http://homepage2.nifty.com/yss/sw5584/sw5584_bdr1ben.htm
Taking P-CAV into account that is quite close to the half of your 4x times.
The reason why the difference is so small for the LG drive is the following.
4x is CLV which means that it burns the disc at a constant speed.
Incase of the 6x writing speed on the LG the drive uses CAV, the drive will start at 2x and slowly speeds up to 6x. From 2x to 6x at the end will result in a speed close to 4x overall. WHich explains why there is no big difference.
It purely has to do with the drive and the writing methods it uses.
As you can see the Matsushita drive does cut the time by allmost 40% on 8x compared to 4x.
Keep in mind the higher speeds are only possible at CAV or P-CAV modes, because there will be to high rotation speed at the beging for the media to burn at 8x or higher.