Hello,
I encountered serious trouble trying to upgrade to 4.0. My server run php 5.6, so this is not the culprit.
Simply put : it didn't work and reverted to normal wordpress commenting. I don't know why, the plugin seemed to be correctly installed, but it did nothing.
Thankfully, I had a zip of the 3.2.8, so I was able to quickly uninstall/reinstall. I had to manually put all the options (no retrocompatibility from export I guess).
I also lost a custom change I made to insert a text above the commenting area (that should become a feature), but that was my mistake. I digress.
Sadly I can't really play day long to try to make the new version work, but maybe there is a way to find what went wrong. I don't really know where to start, but feel free to ask the information which would help you.
Please make sure you've deleted all caches after update
Once the v4 is activated, navigate to Comments > Forms and make sure there is a Default Form and it attached to Post and Page content types.
If you still see any problem please explain it, we don't follow you if you only say "it doesn't work". Specify please what exactly doesn't work, leave some Screenshot.
And if you need to downgrade , you can find all versions here: