I am running Windows 10, and am using a 4k monitor. I have a secondary 1080 monitor.
I cannot change the number of pictures in grid view to be more than two across. it does not matter if I have the second monitor active or not. I have tried the = and - keys, as well as the scroll wheel on the mouse.
it used to work properly, however I can't remember when it stopped working.
You can control the number of images visible in the grid view through the thumbnail slider which is there in the toolbar.
Click on View menu> Show toolbar > thumbnail slider would be available just above your filmstrip.
that works. thanks!
Glad to know you got it working.