There is only X amount of seats left in that particular booking category (X being 1 in your case)... when you check on AC.com (or most travel sites for that matter), it gives you the lowest fare bucket which has enough seats left for your entire party.

You can book your passengers individually to take advantage of the lower ticket price, but you're not going to get 4 tickets @ $1469.... if there's only 1 seat left at that price, you're only going to get 1 seat at that price... and so on.