The four-hour sale lasts a limited time--it's available only until 1 PM Pacific today, April 7. It's always possible that other retailers (like Amazon) will price match it, but if you want to ensure you get your hands on the game at its lowest price to date, you should act fast.
Microsoft recently announced that Sunset's price had been lowered to $40, coinciding with the release of its final DLC pack, Dawn of the Rise of the Fallen Machines.
Sunset Overdrive was released last October as an Xbox One exclusive. Developed by Insomniac Games, best known for PlayStation-exclusives like Ratchet & Clank and Resistance, it's an over-the-top open-world game where almost everything in the world can be used to jump or grind on. The weapons also help to set it apart from other games, with bowling-bowl launchers and TNT-strapped teddy bears being among the lineup.