@ahoeben Ok, here are the steps (with my win10 64bit pc)
1. Start Cura 4.0.0 beta
2. At preferences go to printers and add a new custom FFF printer
3. Go to this printer machine settings (default is 1 extruder) and there is the extruder tab
4. Quit Cura and start it again, the extruder tab is gone now! This is the issue.
5. To get the tab back go to machine settings and select i.e. 2 extruders. Two extruder tabs appear
Sorry for my bad english
Regards Patrick