Christmas cheer is not required for watching Four Christmases. The film takes place around the holidays, but the plot is only seasonal to the extent that it required the protagonists to visit all four branches of their disparate families. Over time the protagonists decide that their relationship is worth investing more seriousness into (the film is similar in this way to Away We Go).
Aside from the phenomenal cast (composed mostly of Oscar nominees or winners, with the exclusion of Vaughn) the film isn't really up to much. It's no Christmas classic.