Well they didn't -really- scrap world scaling, they just did it in a different, better way. As you level, the higher level creatures unlock and appear in place of their lower level brethren. Not fun getting giant radscorps chasing you near Goodspring when you're not paying attention. lol
Same way that they did it in FO3. When you place a spawn point in the world (in GECK) you can pick what type of critter appears there as well as what level range (more or less that's how it works).
So you could create a spawn point that always spawns low level critters like flies, mole rats, dogs. Or you could create a spawn point that always spawns super mutant overlords. Or something in between that is more scaled based on the player's level. It's up to the level designer to choose what sort of spawn points they want to insert into that game cell.