You get whatever you would normally get for a 4th level character, including the feat or ASI. The only way you wouldn't get the ASI is if you multiclassed.I guess I am getting the Mobile feat and -2 Con +2 Dex?
The Mobile feat looks awesome, though the stat mods mean I either have to trade this to someone or change my character idea. We're making 4th level characters? Does that mean we get an ASI or Feat (assuming we don't multiclass)?
Correct.The player gets to decide which stats to decrease right?
Correct.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.