I don’t know if you can get this where you live, but I keep my fridge stocked with a case of Saranac Trail Mix. It contains 4 each of:
Pale ale
Adirondack Amber
Pilsner
Black Forest
Black & Tan
Lager
A little something for everybody!
BUT if not, my 4 are:
Michelob Light- Best light beer out there right now
Guiness- for dark beer lovers
LaBatt’s Blue- Heavier Canadien beer
I would also have one really exotic Euro beer just so your friends think you are really cool. Blue Moon or Bass Ale would not be a bad choice.
Don’t go with Heineken. Thanks to the Bud commericals, it has been painted as a geek beer right now, and besides, it’s overrated.
You’ll satisfy everybody with this classy, yet accessible mix!
I’d also keep a six pack of Mike’s Hard Lemonade and Woodchuck Cider in there for those pains in the asses (usually women) that don’t drink beer and insist on you provide expensive mixed drinks for your party (though I cure that my making my parties BPBYOL- Beer provided, Bring Your own Liquor). Besides, chicks seem to like those 2.
PARTY ON, DUDE! 
I grew up not far from the Sierra brewery, and visited every change I could. While I agree it’s a excellent beer, I think it doesn’t travel so well. Also, the pale ale has never been one of my favorites. Try a domestic (Wisconsin makes some good ones) Hefeweizen or Kristallweizen. Especially good now that summer’s here. Very refreshing with a slice of lemon.
In the fridge now:
JW Dundee’s Honey Brown
(an inexpensive lager with decent a.b.v)
Dominion Ale
(brewed here in Virginia, so extremely local)
Murphy’s Stout in cans with Widgets
(decent stout, slightly smoother than Guinness)
Boddington’s Bitter, again in cans
(excellent approximation of proper 80 shilling)
and Lindemans Framboise for my wife
(Belgian fruit-infused beer.Not an imitation. She swears by it in the summer. I think it’s OK, but will drink Bud before I’ll voluntarily drink it.)
Hmmm, hard to think of four that would suffice by themselves. I’d choose an IPA for myself (either Brooklyn or Harpoon), Guinness or Bellhaven for those who like beers with widgets (and I don’t know many people who would hold out for a pub-poured Guinness at a party where it’s free), Sam Adams or Bass for those who like darker beers, and something lighter (Molson Canadian is good for this) for those who want to down a few, or who like lighter beers.
Of course, I prefer the sort of parties where there’s just a whole ton of cheap beer, and I don’t have to drive home.