> I think that should be FAQ but I didn't find an answer on this.

That is in release notes, see page 6.

> Is Jaybird faster in Type 2 Mode than Type 4 when running the DB on
> the local Machine

If you access the database witl jdbc:firebirdsql:local:..., yes, otherwise -
slower.

> ? Do I lose/get more Features with one of the two possibilities ?

Featurewise they are the same. The biggest drawback is that you load an
external shared library in your JVM process. So, if it is buggy and crashes,
your complete JVM crashes. Also check release notes for information about
multithreaded access under Linux.

Roman