I can assure you that I have done nothing to break alt-tab, V4 sets up the keyboard for you at install time and that should be good. So if its not working I must assume that you've done something odd such as restored a backup that messes with the keyboard settings or followed some out of date instructions to self-break the keyboard settings?
Your keyboard settings should be in /etc/default/keyboard (eg XKBLAYOUT=gb)
You should have fn+shift+[b|m] altering your keyboard brightness, alt-tab switching active apps, and other similar shortcuts etc.