FYI on upgrading from MK3.9/MK4 to MK3.9S/MK4S  

FYI on upgrading from MK3.9/MK4 to MK3.9S/MK4S

I just upgraded my working MK4 to an MK4S.   Firmware 6.1.3 loaded before the update.   Some things that you aren't told.   

The wifi update changes  your Wifi Mac Address.   It will most likely change the IP address in the router.   Which means if you use HTTP://192.168.xx.xxx to address the physical printer in PrusaSlicer, you will have to edit all the Physical printer definitions.    You have to add the MK4S to the configuration so you are in there doing updates already.   Also update your Prusalink web address.  

An alternate to http://192.168.xx.xxx is to use the hostname set in the  MK4S printer.  Example http://MKx-hostname   The hostname can be set in Settings, Network, WIFI, hostname.  If you use the http://hostname for PrusaSlicer, it doesn't matter if the IP address changes. 

Prusaconnect did not connect to the printer after the upgrade.  Had to add the MK4S as a new printer.  When I tried to configure the MK4S for a new PrusaConnect using Menu :   Settings, network, Prusa Connect, Add Printer to Connect.  It failed, over and over.  Finaly I turned off Prusa Connect in the menu, exited the Prusa Connect menu.  Then went back in, enabled  Prusa Connect and then the Add Printer to Connect function worked right away.   Disabling/re-enabling the Prusa Connect in the menu seams to have cleared old data enabling it to work.  

The new cover for the nextruder did not have a spacer washer.  I looked at the old cover and the spacer washer in the cover did not appear to have been rubbing on the gears, so I guess it is not needed.  

When setting the type to MK4S under hardware, don't forget to enable using a Sock in the Hardware settings also.  For those that have not been using a sock already. 

I already had the MMU3 Nextruder upgrade parts in place, I kept the MMU3 versions and did not use the new PCCF tensioner, idler and main plate pieces.