I had this same issue on my Fold 4 (20 month old!) WiFi would just turn grey and not actually switch on. Got in touch with Samsung via the web chat, get the usual do this do that including factory reset etc.. Eventually Samsung sent a courier out to collect my phone. Phone was with them for a week. They replaced the main screen and both batteries. Not sure what that had to do with the WiFi but it now works! Oh and it was FREE! After some light geeky reading I read that the flex mechanism has a part to play in knocking the WiFi off and somehow it's attached to the inner screen. Anyway it's all sorted now. Did I say it was FREE? Anyway that was my experience. Said Anyway a lot now... Anywho's... That's a what happened to my 20 month old fold 4 
