According to Shimano Europe, Dura Ace 2004 has "gone through a complete redesign...shifters not excepted."
The Shimano statement says particular attention was paid to the shifters because "riders spent huge quantities of time in direct contact with them and bike operation is handled through them."
The angle between the hood and the handlebar has been reduced, offering more hand comfort, and it's claimed stress points have been "practically eliminated."
The hood has been reduced 5mm in circumference, which "makes the shifters suitable for all hand sizes and offers better gripping to all riders."
A modified pivot improves shift lever access and there's a redesigned internal mechanism.