I wouldn't let 4 (or 6 or 8) teen boys go with no adult supervision, or 4/6/8 teen girls. I know your daughter was kind of an after-thought so is this just to get help with the expenses? I doubt it! It's interesting that others backed out - I wonder if parents put the kibosh on that.
The whole liability issue bothers me. There's the sex but there's also the drinking and the smoking pot and who knows what else. There's the whole issue of a rental to a bunch of teens (so someone's family either owns the house, or someone did some fancy falsifying to a real estate manager about who's renting this. The post-college short rentals are bad enough, but a post-high school rental with no adults? That's a huge red flag. (And I'm not a prude - I know my son went on a post-college senior week thing and what all had to happen to make that possible.)
I'm curious about the limo. Where did that come from, and why? I think of that as a prom/bachelor party thing where there's drinking or just kids who are too young to drive others at night. Why hire a limo to go to the shore? Is someone paying for a limo for the whole weekend? If not and the limo is just to get them there and back, what's the weekend plan? To sit in the house and just walk to the beach during the day? I'd investigate this a whole lot more. I'm guessing you don't have anywhere near the full story. I think you need to talk to whatever adults have been involved: the homeowner, the person who signed the rental contract, or a chaperone.
While it's not impossible for teens to date for 8 months without having sex, it's also not uncommon for teens to fail to tell Mom and Dad when they are.I hope you will have more than one talk with her about this. If you daughter thinks that condoms are a reliable for of contraception by themselves (even assuming the guy knows how to use one properly - which many do not), she needs to be better educated.