That's because the month for December is 12 and you're adding 1 to it to make 13, which is not a valid month.
Try something like this:
(
(Month({ucase.cs_date_filed}) < 12 and
{u_case.cs_date_filed} in
(Date (Year({u_case.cs_date_filed}), Month({u_case.cs_date_filed})+1, 1) -4) to
(Date(Year({u_case.cs_date_filed}), Month({u_case.cs_date_filed})+1 , 1) -1)
OR
(Month({ucase.cs_date_filed) = 12 and
{ucase.cs_date_filed) in
(Date (Year({ucase.cs_date_filed}) + 1, 1, 1) - 4) to
(Date(Year({ucase.cs_date_filed}) + 1, 1, 1) - 1))
)
Note where I've put the red parentheses - those are required in order for this to work correctly.
-Dell