Many people think that there are four weeks of Advent. Well, only in some years does the Season of Advent last four full weeks. In most years, Advent is a little shorter, depending on which weekday December 25 (Christmas!) happens to be that year. The more precise answer to the question posed above is that there are always four Sundays of Advent, but that the liturgical Season of Advent can be between three and four weeks long. The following table gives the precise dates for the current year, and several past and upcoming years.
The table and information was compiled by Fr. Felix Just, SJ who has an amazing website that you should explore!