There are not 4 weeks in a month. 4 weeks is 28 days, which is a bit less than every month except a non-leap-year February.

A closer estimate would be 4.5 weeks in a month, which would be $45 a month times $12 = $540, fairly close to your (correct) $520 computed by just using the number of weeks in a year. (Although, strictly speaking, there are 52 weeks + 1 or 2 days in a year, depending on whether it's a leap year.)