There are a ton of outdated travel "hacks" you should definitely ignore.
The reason why you can save this way is because "airlines often sell multiple fare classes at different prices, with a couple seats in each class," write the deal experts at Travel Zoo. "If there's only one seat left in the lowest fare class and you search for four seats, most automated systems will show you the highest fare class for all four tickets."
That means if you search the tickets separately and then as a block, you might find some wildly different rates. And remember: If you do decide to book one at a time, make sure there's no per-ticket processing charge that would offset any savings—especially if you're booking through an online travel agency instead of through the airline. You'll also want to select your seats in advance, so you can still sit next to your travel buddies.