Minimig AGA 060 RTG

[Louis Dias]

You may be right, but a lot of manufacturers state 4:3.
 
Here's an example:
http://www.viewsonic.com/products/desktop-monitors/lcd/graphic-series/vg932m.htm
 
But if I do the math, it is closer to 5:4 than 4:3.

It could still be 4:3 if the pixels are not square.  You'd have to physically measure your viewable area to verify this.