I know this a strange layout(I have a complex reason for running it this way).
Just having shotcut running on screen 1 solves it(As it makes the menu appear correctly on screen 1), but that’s just a workaround.
If I run shotcut on screen 4 the menu disappears altogether(Can’t be seen anywhere).
I tried playing with two video monitors recently and posted my findings here:
I came to the conclusion that, although distributing Shotcut’s windows in alternate positions across multiple monitors is possible, it seems to work best on one video monitor. In an attempt to answer your question, as long as the Filters window is open and not obscured by anything else then the Plus option to add filters ought to be visible.
This is addressed in version 17.11 by embedding the filter menu into the Filters panel instead of being a popup. Besides, I want to give the filter menu more space to add features soon like categories, search, and possibly a preview thumbnail. And I do not want a popup to be that big nor do I want a dialog or separate panel. Check it out.