Four Christmases is funny, but a certain number of flaws make it less appealing to me.
First off, Vince Vaughn uses some of the same tricks he did in Wedding Crashers, but at least he's toned down a bit. For the most part, though, the acting is good, if purposefully over caricatured, even Vince. Reese Witherspoon is obviously leading the pack. Robert Duval is not far behind: you just love to hate his character.
Second, the scenario gives us two characters with out of the box ideas at the start, but we loose them by the end. Well, not completely, however, it's enough to put a damper on my appreciation.
Lastly, there are some illogical sequences of event in the story, especially at the start.
Despite all that, it's still funny, and the premises are quite original. If they could only have continue in the same vein, the final product would have been great.
If you wanna a see a positive Christmas story with an interesting setup that requires very little thinking, I think you should go for it.