thanks, Windows+P does the job
Choosing projector only is a quick way to turn off monitors 1,3,4 and then everything is just on my high screen as I wanted. Well not quite what I was after, but it'll do.
Only problem now is my sound output is sent to the inbuilt speakers in monitors 3 and 4, and they go quiet when their input is cut (got rid of external speakers in a tidy up operation some time ago). Ah well, can sort that out with extra wire to top screen or to my boombox.