One "under the hood" change is that Ozone 4 is optimized for low-latency DAW environments. Immediately visible is the improved interface look with "spill out" sub menus such as the new Preset Manager window. In fact nearly every menu has clickable submenus that overlay the plug-in's main GUI--this is a great way to explore the internal capabilities of deep plug-ins like Ozone 4 in an organized manner. The enhanced Intelligent Loudness Maximizer mode allows for louder mixes with less downside effects like pumping and "hole punching". You can control the recorded ambience--the sound stage width and size with the new mid/side processor. I use this to bring up or down the center of stereo mixes--like raising vocal levels or reducing heavy bass without using equalization or filtering.

The aforementioned updated preset manager features MacroPresets--a greatest "hits" of preconfigured mastering setups named by the exact sonic issues they address. These presets are ideal starting points--I would get close and then tweak to suit the needs of the particular song(s) I'm mastering.