Excuse if this is a stupid reply but are you using RW or R media. If it's RW currently you will only be able to burn at 2X max -- you will probably find running
Nero or some other tests your burn rate is actually about 1.85X
If it's DVD-R you should be able to burn at 4X provided you have 4X media AND the hardware device (i.e your burner) supports it.
If you are using proper DVD-R 4X media and your device is rated at 4X for burning DVD's the only thing I can siggest is that you see if there is a firmware upgrade required.
(I'm using 'Doze XP as a guest running vmware on Linux Mandrake 9.1 and 4X works on LG burner -- can't tell if Mac supports 4X or not).