I want to know if you can use a guest account on Xbox Live to add a second person to play horde with or do you need a second Xbox Live account to do this?
Splitscreen is supported in all the coop modes for Gears of War 4 and even online.
You should not need a second Xbox Live account. I've never seen a game do that for splitscreen. Usually, the second player assumes the first players name with a number in parentheses next to it indicating which player they are.