This isn't even a foreign concept when it comes to gaming. Those 4-6 PCs are going to be the pivot points for a major story, they are going to be shot at, stabbed, chased from cities, have to fight dragons, risk their life, and probably watch a lot of good friends die along the way. How lucky does that sound? There is a reason that "May you live in interesting times" is a curse, and it is one that PCs have in spades. That being said, many games do lack that little bit of extra spice. Most GMs realize that PCs are special, they're unique, the universe will work things into their favor. But Karma is a royal pain in the ass, so where is the balancing out challenge?
Now, I'm not saying to just drown your players in bad luck mind, however a bit of bad luck here and there is to be expected. If there are supposed to be 4-10 guards, odds are it'll be closer to 10 than 4, why? because fate loves to challenge the ones it has picked to be special. The kid the characters are supposed to be protecting runs off, of course he's going to choose the left fork towards the dragon cave over the right fork towards the pillow factory. These are, in a way, relatively small things that happen, but they happen, and they help make the story more interesting as well by adding to the challenge.
You do still need to be careful, adding Bad Luck to the groups trouble is a careful thing to do, in a way it is like adding salt or sugar to something. A little is good, more can be great, but there is a very fine line between "perfect" and "way too much!". So while you are doing it, watch your players, make sure you aren't bogging them down with challenge after challenge, give them some good breaks to. Remember, you're not looking for a dose of bad luck, you are looking for a healthy dose of bad luck. Have fun with it, make sure they are too.
P.S. sorry for the shortness of today's post, I've been meaning to write SOMETHING about bad luck for a few days, and to keep my schedule for here, but school is currently wrecking me. I figured something was better than nothing. May return to this topic later.