In case you didn't know, the change-log is within the readme.txt that pops up when you first open UTAU. It's in the folder as well, if you want to look at it.
The most notable additions are the automatic VCV conversion and the ability to show the waveform (if that's how I'm understanding the Japanese correctly). However, next to these, in parenthesis, says, "registry benefits" which basically means you must have shareware to have the function. (romanized, "rejisuto tokuten", with "rejisuto" in katakana, and "tokuten" in kanji.)
In addition, the change-log states that the UI has changed from Japanese to English, the rendering engine has been improved (greatly), there is a zoom button to the right of the horizontal toolbar, and if I'm reading this correctly (I haven't seen the inside of the program yet), then the UI is larger than the previous one.
There were no publicly released versions between 4.0 and 2.76, so there are no additional changes noted between them.
[30 minutes into making the post]
Okay, so, the resampler! It's better. It generates a lot of breath-like noise in the background. So that must have been improved. Also, general results for the resampler are closer to TIPS in terms of final timbre, but still retain the quality of resampler's older version, if not only a little bit. However, sometimes vowels are scratchy, like the previous incarnation of it. There is a new glitch with it though- BRE 0 is no longer a saving grace and will cause some voice-banks to soft-clip with the new resampler, as well as pop on some note transitions. The result sounds fine with music (assuming your mixing is decent), but it might bother some people (like me) when working with it raw. Also, the new resampler is better at resampling higher ranges, but the lower ranges seem to have mild difficulty (popping and soft-clipping is more apparent in the lower ranges if you're on BRE 0).
In addition, a progress bar made of ASCII has been incorporated into resampler (I'm not sure if this was added through wavtool or the resampler itself, but either way, it's still cool.)