Version 4.0 now allows you to set a separate filament diameter for each extruder. So if you want to use 1.75mm filament in one extruder and 3mm filament in another, this is now very easy to configure. Just click "Edit Process Settings", go to the Other tab, and use the "Filament Properties" section in the top right of that window. Choose the filament toolhead index that corresponds to the extruder that you want to adjust, and then enter the diameter for that tool. Pick a different tool from this drop down if you want to adjust the settings for a different extruder.
PS - you can check which toolhead your extruder is using on the Extruders tab of your process settings. Just click on the name of your extruder in the left-hand list and then check the "Extruder toolhead index"