I don't play enough Fool's Gold / Thief games, but I play plenty of Apothecary, Scrying Pool, Herald, or Mystic games where I can afford to gain a nonterminal 4.
You shouldn't buy Scout just because Apothecary, Scrying Pool, Herald or Mystic is in the game and you can afford to gain a nonterminal 4. You need to be able to utilize the fact that it's a non-terminal Action that costs $4 and is named Scout somehow, you need to be able to utilize the deck reordering somehow, you need to be able to utilize the Victory card drawing somehow and you need to be able to gain it for entirely free (i.e. the only other alternative would be nothing at all), and then it's just a super little bit better than nothing. If the only thing it's doing for your deck is helping your Mystics, you should trash any Scouts that you have gotten somehow.