Looks like FH4 disconnects that happen while running on the XB1X in performance mode (60hz) are gone on the Series-X running the optimized game. The disconnects most frequently happen when in a #Forzathon or match made race. Restarting the game every hour or so on the XB1X seemed to help. Also so far I have had the game lock up when transitioning to the menus only once on the Series-X. These lock ups happen occasionally on the XB1X. When this happens the only fix is to restart the game.
Also FH4 running on the Series-X w/ the optimized game crashed once now. Transitioning to the 3rd round of a #FORZATHON live event the game exited to the dashboard/crashed.