Fox is desperate for a sure-fire franchise. Universal rakes it in with Fast & Furious and Jurassic Park. Disney has Marvel and Star Wars. Lionsgate has Hunger Games, Twilight, and Divergent. Warner Brothers has Harry Potter and DC Comics (snicker).
Fox wants in on that game and they're obviously hoping Avatar will do it. Until Deadpool, the Star Wars prequels were their biggest movies. They lost Star Wars and Deadpool is a Marvel property, so who knows what share Disney gets or even how long they have the license? It's the same with Fox's X-Men movies. Disney's probably gets a cut of that, right? Or at least gets it back at some point? So, yeah, they're looking for a mega-franchise to call their own. And what else are they going to do, start one from scratch? Hence, four planned Avatar movies.