I have yet to uncover any detail info other that provided in the release notes.
I was finely able to uninstall XTU 3.2 after a number of failed attempts that produced uninstall errors which prevented the uninstall attempts from completing.
I re-installed XTU 4.0 without errors and it seems to perform as anticipated during the Stress Testing.
However, I have yet to perform any tuning.
To answer your question, the XTU 4.0 includes additional features for application performance. You can check the release notes for both versions at the following websites:
Our recommendation is using the latest version of the application.