The question starts with a false premise. Four weeks does not equal a month. Only February has exactly 4 weeks on a non leap year ( it has 29 days on a leap year). Every other month has 2 or 3 days more than 4 weeks. In most cases its practical to estimate that a month is approximately 4 weeks. however, in most cases extending that estimation for 1 year (12 months ) creates too large a gap ( Approximately 4 more weeks ).
That is why a year is better estimated as 52 weeks.