Question about dvd burner

Hello,

First of all, excuse-me if my english is not very good... and ny the
way, this is my first post here!

So, i've buy this week end a dvd burner, the LG GSA-4040B and 'ive
upgrade the bios from ver A300 to A301 (direct from LG website).

I've buy blank dvd too, from many company (no brand, Princo, Maxell
and Mitsumi) and all these blank dvd are certified 4x.

The problem, is nero does not detect the blank media at 4x, but only
2x! I can burn dvd-rw at 2,4x, 1x, but not 4x...

What can be the problem? The place where i've buy the burner tell me
it's only because the burner detect the speed for the blank dvd.

But why all company certified the blank dvds at 4x but cannot go
higher than 2x?

Anybody have a solution for this? This is a normal situation?

Thanks alot!