F4 should certainly still be available. Does double clicking on the canvas still work?
It is possible that the F4 short-cut has been changed. Can you check in the GH preferences Interface -> Shortcuts settings to see if the menu item for Edit->Create… is still F4?
First time when I open WIP and press F4, the entire screen is “magnified” (zoomed in extremely much), then I escape from that, and F4 is dead altogether after that.
Sounds like something else is using F4 as a hotkey and intercepting the event. Peculiar it’s on Rhino6 only, but… Do you have any screen recording/screen capturing/presentation software installed that might have system wide hotkeys?
You could also change the F4 shortcut in Grasshopper to something else like F5, or maybe something involving Ctrl+?.