Please see:
b1: https://dev.blesta.com/jira/secure/ReleaseNote.jspa?projectId=10000&version=10603
b2: https://dev.blesta.com/jira/secure/ReleaseNote.jspa?projectId=10000&version=11001
b3: https://dev.blesta.com/jira/secure/ReleaseNote.jspa?projectId=10000&version=11002
b4: https://dev.blesta.com/jira/secure/ReleaseNote.jspa?projectId=10000&version=11003
b5: https://dev.blesta.com/jira/secure/ReleaseNote.jspa?projectId=10000&version=11004
b6: https://dev.blesta.com/jira/secure/ReleaseNote.jspa?projectId=10000&version=11005
I sadly can't get the release notes on Jira for the full v4 release :S but that should tell you.
The MinPHP upgrade makes future updates much better as it can support the new domain manager functions etc. It also is ready for PHP7.0 which a lot of people need as 5.4, 5.5 and soon 5.6 are going to be outdated and EOL.
There's a long waited mass mailer included in 4.0.0 and it also has a lot more features I can't think off the top of my head. The dropdown menus is what people asked for as they found the submenu was hard to navigate with.