I don't think anything essential is ever cut, rather you get more and better tweaks to existing features, version by version. I think one of the things in XR4 that may prove most useful is the 'multiple parts' facility in the NC manager, but you won't get that in the lower level packages. But the high speed machining technology in pocketing has continued to evolve as well. Without knowing the details of what kind of work you face doing, it is not possible to forecast other benefits, but I know I hate backtracking to use the earlier version of nearly any software.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)