It just happened again 
Opened PL4 first, than Nikon Transfer 2. Transered photos from the camera. Means, NT2 creates a new folder and voilá. But the file browser (PhotoLibrary) of PL4 doesn’t show it. So I have to restart PL4, so that the new folder appears. Is there a better way for this? Cannot find anything about it.
P.S.
I think I found the reason why. Though it does not makes sense at all for me. The folder is in my “favorites”. When I go through “Devices” to this folder, it is there.
P.P.S.
Indexing is no solution, takes to long.