Typhoon I think Golfnow.com or any other tee time booking website (or the golf course too, for that matter) should make it clear that they WILL fill the remaining spots if you book a twosome. My wife and I run into this in Florida all the time, but (since I worked at golf courses several years) I always assumed that would be the case.
The only way to insure that you will be allowed to play with only two people alone is to pay for all four spots, I guess.
And not many people would be willing to do that! BTW, for the most part, Mrs. Fatshot and I have been paired up with perfectly pleasant people 95 % of the time....!