Weird multiple display issue

[Thomas Cameron]

Howdy -

I have a multi monitor machine. The displays are set up like this:

+ + +
     +

So I have three large monitors and a smaller tablet below the far right 
monitor.

The weird thing is, it seems like the desktop THINKS my screen looks 
like this:

+ + +
+ + +

So what happens is, when I do something like use a background image 
across multiple monitors, it stretches it so that half the image shows 
up on my top three monitors, and about a third of the bottom of the 
image shows up on my small monitor down to the right.

Also, and this is the real problem, when I choose to rearrange my 
desktop icons, it distributes them across the left side of my left 
monitor and down across the nonexistent left bottom screen it seems to 
think is there. So my desktop icons disappear. About half of them appear 
to be placed under the leftmost monitor, but there's no monitor under 
that top monitor.

I've posted pix of what my desktop looks like, and what I see when I 
capture a screenshot. https://imgur.com/a/gUBsgML

In the screenshot, I have Chrome maximized on all four physical screens. 
As you can see from the screenshot, there is empty virtual desktop space 
below the left and center physical screens. You can see that my desktop 
icons are in that empty space. But I can't see them on the physical screens.

Anyone know how to fix this so that my desktop icons don't disappear off 
the bottom of my screen?

Thanks!
Thomas