Help Problem On SELECT Cycle Backward

need help on logical SELECT cycle



Code Snippet
SELECT empID, unit_date, unit, ISNULL(NULLIF ((unit + DATEDIFF(mm, unit_date, GETDATE())) % 4, 0), 4) AS new_unit
FROM dbo.empList




i have this code
this code change the value field "new_unit" evry month from 1 > 2 > 3 > 4
like this evry 4 month it return to 1 >2..........
------------------------------------------

if i put
unit_date = 01/05/2008
and unit=1
than new_unit=1
IT OK

but

if i put
unit_date = 01/04/2008
and unit=1
than i get new_unit=2

but it should be 3
it dont go backward ok



if i put
unit_date = 01/06/2008
and unit=1
than i get new_unit=4

but it should be 2
it dont go forward ok !

TNX