Lots of changes to DPP, runs faster too. One of the first things I notice is that triage is improved - the Quick Check mode now allows you to scroll through at 100% magnification (or a handful of fixed mags between 12.5% and 400%), rather than only the full image or 50% of v3.x. The overall workflow is closer to 'mainstream' RAW converters - all the tools are applied in the main editing pane, as opposed to v3 where cropping/horizon and clone/stamp were standalone 'modules'.
I'll stick with DxO Optics Pro myself, but DPP v4 does look like quite a substantial upgrade for DPP users. Those new to Canon RAW file processing may find it pretty usable (unlike prior versions, IMO).
Splash Screen:
New Dock Icon for Mac OS X: