In addition to the Range Masking tools already mentioned, there was also a change to the way the Auto Mask generated it's masks. Because of the way the masks are generated and stored, there was no easy way to shoehorn those changes into PV3 (2012).
The old Auto Masks had some problems in the way it felt with noise in captures and the mask generation. The new Range Masks were designed to address those problems but, there's a limitation in the mask generation–for example you can't add a Color Mask and then use the Luminance Mask option to erase certain portions. But you can combine either Color or Luminance Masks and use Auto Mask to add to or subtract from the Range Masks...You'll see how that's useful once you get used to using the new tools.
And "somebody" yelled bloody murder when Range Masks were going to replace Auto Masks...so the engineers went ahead and fixed the Auto Mask generation and added Range Masks as well. The combo is even more powerful...
The reason for the PV bump was because the old Auto Masks couldn't be easily updated and the format of the way masks are now generated and stored changed, so that's why PV4 (Current) was added...