You're probably going to have a difficult time with this, as the fourth monitor is at a different resolution than the others.
The simplest way would probably be to edit the photo manually to get the fourth monitor to display properly.
I have a 1920x1200 and a 1280x1024 and when I display an image on both, what I do is create a 3200x1200 image in my photo editing software. I then use the left 1920 pixels for the left monitor (1920x1200) and the right 1280 pixels of the image for the right monitor. I then scale the image on the right hand side down to 1280x1024 and put a blank space along the lower 176 pixels of the right hand screen.
So my image ends up looking like this:
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I realize that it's bad ASCII art, but you get the idea. You'd have to make your image manually to do what you're trying to do.
The alternative is to "suffer" with your 3x 1080p monitors and quit using that 1280x1024 monitor.
An advantage to getting rid of that odd-resolution monitor is that your video card will consume less power if you keep all resolutions equal. If you use an odd resolution, your video card will remain in 100% power mode, P0 state and will not power down to idle P12 2d state or P8 video state unless you use 3rd party programs.