CASE
    WHEN col1 > col2 THEN SUM(col3*col4)
    ELSE 0
END AS some_product

select ...
from (select ... , CASE WHEN col1 > col2 THEN SUM(col3*col4) ELSE 0 END AS some_product
   from ...
   group by col1, col2 ... ) T
group by some_product ...

with T as (select ... , CASE WHEN col1 > col2 THEN SUM(col3*col4) ELSE 0 END AS some_product
   from ...
   group by col1, col2 ... )
select ...
from T
group by some_product ...

Select Sum(Case When col1 > col2 Then col3*col4 Else 0 End) as SumSomeProduct
From ...

Group By Case When col1 > col2 Then col3*col4 Else 0 End

Select SumSomeProduct, Count(*), <other aggregate functions>
From (Select <other columns you are grouping By>,
      Sum(Case When col1 > col2
          Then col3*col4 Else 0 End) as SumSomeProduct
      From Table
      Group By <Other Columns> ) As Z
Group by SumSomeProduct