Try this:
SELECT ((DATEPART(d, @input) - 1) / 7) + 1
We don't care about the actual day of the week, just the number of days past the first of the month (which is the day value - 1). Then an integer divide by 7 to get the number of weeks past the first of the month, and finally + 1 to have it start at 1 instead of 0.