You don't sell somehting if you think it is going to grow. you sell it if you think it is going to drop. WIth the USADA scandal, i doubt the fertittas had high hopes for the future.
honestly if ufc keeps going at this rate i doubt it will be around by 2020
That's not even slightly true. Many people cash out investments, not because they don't think it's going to grow, but because they have something better in mind (or they might just be tired of it)
The Ferttitas have cashed out in many of their ventures lately, or have at least consolidated what they have and minimized their personal exposure.
When this first was finalized someone around here made a good guess at what could be going on. Vegas is getting it's first major sports franchise and there has always been talk of them getting an NFL team. There's an outside chance the brothers want to make a bid to have that team, and having $4 Billion dollars on hand makes them a major player, as it will cost $1 Bil or more just to get the license from the NFL, but afterwards its seriously an ability to print money, as the NFL is almost a sure bet for even the crappiest of teams.