I'm sure David Arbogast will be able to answer what the improvements are with the MB IV but if I recall, the black material on the inside of the adapter has been improved and the ability of tilting and shifting tilt/shift lenses to their limits has been addressed too.
Your statements are correct Gene. They made the hole a bit larger to address the lens movements better and they improved the interior finish to better inhibit the internal reflections that users were having. I flocked the interior of my mk III, so it will be interesting to see if there are any internal reflection issues with either.
Sadly, I just haven't done the testing to make a definitive statement one way or another on the value of the mk IV over the mk III with TS-E lenses - I might take a moment this weekend to do so. Bear in mind, though, that Metabones specifically modified the adapter to address the real complaints that TS-E owners were expressing.
The shot below was taken with a fully-shifted TS-E 24 II and the mk IV adapter and it has some terrible issues at the top: It is blurry and badly so. I am fearful my adapter may have a poor fit, which may cause excessive blurriness in the areas of extreme shift. I know the lens is softer at edges of extreme shift, but this looks especially bad (to me).
[Sincere apologies if this is a re-post - just posting here with regard to the Metabones IV discussion]
Queen City by David Arbogast, on Flickr