Um... 4:3 is suppose to have black bars. The only way I could see you fixing this is playing in Windowed Mode, or buying a 4:3 monitor to play on.
I use to play lot's of CS:GO, and it's my personal opinion to play on 1920x1080 or higher. The 4:3 thing just seems to be a myth to me.
If that means anything to you.