4 Great Unaged Mezcals for Cinco de Mayo
Tacos and margaritas on the fifth of May are no more Mexican than green beer in the middle of March is Irish. But if you're looking to put a shot of authenticity into your celebrations this Saturday, you should really consider making it mezcal day. More

@Sov Thanks for your comment. This is actually a pretty in-depth topic, which is why I didn't get into it in a review of several whiskeys as I wanted them to be the focus, but perhaps I should write a longer piece on all the different types of whiskeys (and whiskies). One could write a whole post on spelling variations alone... but let's sort this out!
Let me put out some quick working definitions (by no means exhaustive, as they get very technical fast):
Single Malt: 100% aged malted barley whiskey from one distillery
Blended Malt (formerly called vatted malt or pure malt): blended single malt whiskeys from different distilleries
Blended Whiskey: practices do vary a bit, but in general this is grain whiskey blended with aged malt whiskeys
Single Barrel (sometimes Single Cask): the product of a single barrel or cask, with some water added to bring it down to proof
Single Barrel Cask Strength: one barrel, into the bottle. that's it.
There are other categories also... bourbon, bottled in bond, canadian whisky, rye, etc, etc...
Tricky stuff. But the fact of the matter is that yes, in general single malt whiskeys are actually a blend of a bunch of different barrels of aged whiskey from a single distillery. This is how distillers are able to achieve a consistent product since, as you correctly note, each individual barrel will have variations in taste and color due to the vagaries of the aging process. The master blender tastes all of his barrels, and using his artful skill in the craft acquired through years and years of experience, she is able to orchestrate the blending of these barrels into an expression that people the world over know and love. The age indicated on a bottle of a single malt is the age of the youngest whiskey used to achieve that expression. So you can really read those as "x years or older."
Also it's not true that "all whiskeys are generally aged at least 4 years." There's been a trend recently towards early releases of a new whiskey, young (6 months!) whiskey, and white dog (unaged whiskey) hitting the market. "Whiskey" is a tremendously broad category - a distilled spirit made from fermented grain. That's all you have to do to make whiskey, aging isn't required. This is precisely why there are all of these subdivisions to differentiate different products!
It is a little unclear in this post, but both Bushmills Original and Black Bush are blended whiskies, while the 10 year and the 21 year are single malts.
I hope that clears things up!