I thought I had this exact same thing (while testing based on your post) ... My burner is also 4x, and NeroLINUX gave me aswell 2.4x ~~~ then it hit me... my DVD-RW media is limited to 2.4x!

I then put in a DVD-R (with 4x max) and NeroLINUX automatically changed the max speed to 4x .... so all I can say is it's working here fine.

Are you sure it's not the media that is limiting your speed? I do have an Asus DRW-1604P though, but these silly mistakes can happen to all ;-)