The solution is to use TOTAL in your Sum expression. See attached application...
The expression should look like this:
Sum(TOTAL <col1,col3> col4)
This takes the sum of col4, but groups by col1 and col3. This will avoid showing the sum in terms of col2. Notice that the Totals at the top of the column is actually correct, not the sum of all the numbers below it!
Also, please note the sort order; I changed it to make the data look like your desired order.