A 9 inch disc is barely large enough to slice cleanly through a 4 inch brick; I’ve no doubt you can do it with a cutter that size but the blade will be virtually at full depth & the motor will be running a big risk of stalling. With an “Arni” sized disc cutter & the power to match, you will cut the opening in minutes; a clean straight cut, it’s literally like slicing through butter; awesome!
I think it would be total madness to use such a cutter (or even the smaller one) from a ladder
by the nature of the beast, it’s weight & power is significant & you need both hands to use (control) it; lord knows what the cuts may end up like! One of the reasons I never attempted the 2 x 1st floor windows myself is that I didn’t have a platform (the guy that did it went to all sorts of checks up there before he made the cuts) &, at that time, I’d never used a cutter that size before. For your own safety, please give it serious thought; one slip & you will probably see your decapitated limb drop to the floor before you do! 
