Are you looking for the measurements of the plate that covers the holes for the springs, and that the tailpiece & bridge sit on?
If so, I am holding a Mustang on my lap as I type this--I've been noodling around on it, while surfing.
While the metal plate is not a square--at the widest points it's aprox 4" x 3.75" or so.
SO if you have 4" x 4" or about 16 square inches--you have space that way. You still need room in the body to rout for the springs.
If you don't know what the body rout looks like--this is from Mr Maxima's Fender Mustang Story--hosted on this very site-
http://www.jag-stang.com/MrMaxima/gpphotos/routes.jpg