A squarish display is 800 x 600, or 4:3 ratio, or 4/3 = 1.33
Same for 640 x 480. 640 / 480 = 1.33

If the max width 1900, divide that by 4 = 475 wide.
So with a client that is 475 wide, we can fit 4 of them in the bottom.
What would be the height for a 475 wide client?
475 / x = 1.33
x = 471 / 1.33 = 357 height

Log into WoW on each of the 4 bottom clients, one at a time.
Go to Video Settings. Set to 800x600.
Turn down all graphics. Set Scale UI to .62 and Turn off sounds.
Log out WoW normally.
Edit the the WTF/account.cfg
Look for the GFxResolution line and change that to 475x357
Save.
Make a copy of the account.cfg into "475x357 account.cfg"

Run the WoW client and see if you like the scaling. If you go into the Video settings again, it will over write your 475x357 custom resolution. So don't go to video settings if you don't need to.

If you're ok with that, copy the "475x357 account.cfg" into each WTF\account.cfg of the other bottom WoW folders.

To get the top wide client working (WoW Main):
I load WoW Main there by itself, go to video settings and choose one of the lowest (wide) resolutions.
Change the graphics options, and set UI scaling down to .62 (left most).
Log out of the game.
Edit the WTF/config or was it account.cfg file.
Look for the GFxResolution line and edit that to what you want the WoW Main client should be.
Save a copy of the new account.cfg file to "1900x725 account.cfg"
Run maximizer manually and see if your WoW Main is how you like it.