I tried a function on this. Worked for the months of this year, but didn't test others.

lnMonth = 3
lnYear = 2001

IIF(dow(date(lnYear,lnMonth,1))=1,1,9-dow(date(lnYear,lnMonth,1)))  && first Sunday in month

gomonth(date(lnYear,lnMonth,1),1)-date(lnYear,lnMonth,1)    && no. of days in month

If first Sunday + 28 days (4 more Sundays) is less < = no. of days in month,
we have 5 Sunday, otherwise there are 4.  So the function is:

IIF((IIF(dow(date(lnYear,lnMonth,1))=1,1,9-dow(date(lnYear,lnMonth,1)))+28) <= (gomonth(date(lnYear,lnMonth,1),1)-date(lnYear,lnMonth,1)),5,4)