Well here's my scenario for S4. Nanoha is injured again on the job and is finally convinced to "retire." She decides to move back to Earth and raise Vivio there. Fate goes on a long vacation to help take care of Vivio and to make sure that Nanoha doesn't over work herself.
In the meantime, Hayate and family + other StrikerS characters are still working for the TSAB. However, a recent case leads to discovering artifacts possibly from Al Hazard... We soon find out that Precia Testarossa succeeded in reaching the forbidden land and is attempting to come back to take over this world with her daughter Alicia...
So.. slice of life sitcom, Fate's past drama, well coordinated magical battles, debut of Vivio as magical girl, and 13 episode series.. ie. back to something closer to A's