Calculate Colum Automatically Update

Is there anyway when I create a column that's a caculation based on other columns that it can be automatically updated if one of the columns is changed, e.g:

function sum(column, row){
return this.getCellValue(5, row) + this.getCellValue(4, row);
}

obj.setCellData(sum, 6);

So, if columns 4 or 5 change, column 6 is automatically updated.

Thanks,