Something that I used to use that is just a tabular implementation of Grant's suggestion:

create table Months (MonthNo int, Month4Period int, Month6Period int)

insert into Months

Select 1, 1, 1

Union All Select 2, 1, 1

Union All Select 3, 1, 1

Union All Select 4, 1, 1

Union All Select 5, 2, 1

Union All Select 6, 2, 1

Union All Select 7, 2, 2

Union All Select 8, 2, 2

Union All Select 9, 3, 2

Union All Select 10, 3, 2

Union All Select 11, 3, 2

Union All Select 12, 3, 2

Then you just add:

, (Select Month4Period From Months Where MonthNo=DatePart(mm,Date)) as Month4

, (Select Month6Period From Months Where MonthNo=DatePart(mm,Date)) as Month6

to your query columns.