Hi,

I'm sorry that I'm not posting any particular experience. I just wanted to add some general information.
The issues with upgrading are mostly related to customizations and some custom applied "workarounds". To be honest, some newly implemented features can suffer from "child" diseases, but we're trying to fix such problems.

The upgrade process is not an ordinary action. Moreover, the internal changes from 3.x to 4.x are quite large, thus more upgrade issues could occur (for various reasons).
We're sorry for any inconvenience during upgrades and we try to fix all problems and issues that may appear. There is also a new policy of cummulative hotfixes for 4.x version.

Currently, are you experiencing any upgrade issue? We will try to help, feel free to let us know.

Kind regards,
Zdenek C.