I haven't been able to stretch to 4:3 (I have 144 Hz 16:9 monitor) after I started using Windows 10. I got it to work with 15.6 beta drivers for a moment, but after shutting down and turning it on again I haven't got it to work. I have the option about scaling the picture to full panel on, but it matter anything, I always get the black bars in CS:GO. I've tried all the drivers from 15.6 beta drivers to the newest 15.11 drivers, but nothing seems to work.
(And just for those, who wonder why I play CS:GO in 4:3, it is because I've played many years with 640x480 streched graffics, because my old computer wasn't able to run them in any better resolution. Now I can't play as well in 16:9 as in 4:3.)