Summing in a VLOOKUP function

I have a vlookup function that is looking up a range of cells.

I would like it to look in the range and then sum up all of the values that meet the criteria.

At the moment it will only bring back the value of the first match it finds in the range, I would like it to bring back the sum of all the values it finds in the range, is this possible?

Example

Vlookup(Jan-08,$a3:$c$7,column 4 values, 0), if Jan-08 is listed in the range more than once, I would like it to add up all the values in column 4 that match.

Thanks

Lesa