Where can I find a list of "what's New" in Base 5?

edit

The principles applicable to LO Base version 4.2 will also apply to version 5.

You will see from the release notes for version 5 indirectly linked here that there are not actually any large feature enhancements specifically directed to Base. The largest enhancements are generalized aesthetic, scripting, and bugfix changes.

On the other hand, there is one large regression in charting in reports, so if you use charts in reports, I would suggest keeping an eye on that bug report in particular before upgrading. Also, I would test your more complicated scripts because I have seen more race-type conditions in version 5 scripts.