@Brittany142 For starters, I'd insist that the other couple make up their minds one way or the other and ASAP. It's certainly not fair to keep you hanging. If they say they're not going, then I'd first message the host to explain the situation, and make sure they're okay with just you two staying. I can't see why they wouldn't be, but if you paid a higher price for 4 guests instead of 2, you can't reasonably expect the host to shoulder that loss, since if your friend cancels and they can rebook the dates for 4 people, that would likely be their preferred choice. So depending on the cancellation policy, and whether extra guest above 1 or 2 are charged at this listing, your waffling friends might have to take the loss.
Then, if the host is amenable, as @John1080 sugested, contact Airbnb to see how they can assist in cancelling the original reservation for 4 (of course your friend would have to be involved, they can't cancel if he doesn't agree to it) and simply move it over to your account , with an adjustment in the number of guests.