Using your example above trying to help me understand. If I were to go to a new country with no vaccines etc. would sitting in customs in isolation for 4 weeks do anything? It's not going to change what will happen once I am exposed to the new environment. If the new hens have something within them that is capable of wiping out my existing flock what are the odds I would notice it during a short quarantine? Especially if it is something that is not necessarily a noticeable illness compared to something that they would experience from a change in environment. Being basically a newbie and never having added to a flock I imagine one of two things. Either the new hens have some type of illness or disease that could be brought into my flock. If that is the case I don't know what signs it would show while quarantined. The other option is there is something that my environment has that they are not used to. If that is the case they are eventually going to be exposed to it.
@TeeMom already covered what to look for.

And no, it wouldn't, because customs in isolation would have you inside, not outside in the flora and fauna. Maybe you have an oak tree but the person you buy them from have maple trees; that could cause a little issue with one of the new chickens. Maybe there's something in your dirt they don't react well with, like a fertilizer or something. Or maybe they don't like the food you normally give your chickens, or the new food upsets their stomachs a bit. And maybe you have something poisonous in your yard (I have Oleander bushes everywhere surrounding my yard, because I live in AZ) that your chickens have tried a bit of and learned it's bad for them, but the new chickens have never seen it, so they try it and get sick. It's better to be safe than sorry - stress, like
@TeeMom said, brings out anything that might've been lurking in them. You might also want to get them better acquainted with you before adding them in with your girls, and trying to figure out any bad habits they might have/have picked up from stress (like feather picking, for example) before they teach it to your existing flock. Once they all get started, it's hard to find out who's the ringleader.

I'm sure it's always an awful feeling to lose a chicken (none of mine have died yet, thankfully), but I'd personally rather lose one I'm in the process of quarantining than my entire flock I'm more attached to.

Even taking money out of the equation (decontamination of the run, coop, new chickens, raising them from chicks cost or buying them as older hens, etc.), it'd be a tremendous emotional toll, at least for me, to loose my entire flock, even my mean roosters.

As for quarantining, if you have time, you could take them out free ranging after a bit, (maybe a week?) so they can learn the territory and you can see if there's anything environmental they react badly to, while you supervise, or if your already established flock free ranges, just let them out in shifts. Then the new girls can go back into the pen/cage after free range time. I'd just try to make sure they each have as much space as possible in the cage, maybe even a bit more than would be necessary with a coop if they'll be in there constantly. Just monitor them as much as you can, and after their stress levels are down a bit from being moved and put in a new pen, they're probably ready to explore a bit, then after another 2-3 weeks, they'll probably be okay to go in - depends when they're not stressed anymore, and when think they're ready. The average/standard I've seen is 30 days, but I'm not sure.