I've upgraded to Aldiko version 4 on Android, and some features are now missing. Why is this?

Aldiko 4 is a complete rewrite of the application. In addition to several new features, Aldiko 4 will offer all the features that were already available in the previous versions of Aldiko – only improved and made more accessible.

We've simply decided to deploy them incrementally. So it is normal that some features from previous versions of the app are not yet available in version 4. They will become available gradually, as future versions are released.