You could try this:

SELECT *

FROM dbo.SomeTable

WHERE ( @Field1 IS NULL OR Field1 = @Field1 )

AND ( @Field2 IS NULL OR Field2 = @Field2 )

AND ( @Field3 IS NULL OR Field3 = @Field3 )

AND ( @Field4 IS NULL OR Field4 = @Field4 )

Provided that the search fields are NULL when unused and non-NULL when used.

CEWII