Somny dru510 speed issues

Question

i have windows xp pro sp1 all updates, sony DRU- something, it's the 4x one, 510. why won't it burn at 4x on 4x dvd's? i have firmware from a couple months ago and dvd decryptor and dvdxcopy seem to only burn 2x even when dvd decryptor says the media is 4x. I've tried Ritek 4x dvd-r and also princo 4x dvd-r.