This should be no problem.
I assume you have a chart with dimension product and as expression a sum of sales or similar.
Using QV11, you could use dimension limits for this.
Check the 'Restrict which values are displayed...' check box and then first option 'Show only largest 5 values'
(Uncheck Others at the bottom part if needed)
If you are using QV10, you could sort by y-value descending and limit the number of values shown in presentation tab to 5.
Hope this helps,