Using Q&A to display top X values

Tricky with Q&A. Here is the definitive guide for asking questions:

https://powerbi.microsoft.com/en-us/documentation/powerbi-service-q-and-a-tips/

What I would do would use something like RANKX to create ranking in your data model and then you could ask your Q&A question "sales reps with rank less than 4" for example.

If you are using a content pack and cannot manipulate your data model, you probably need to use something like:

"sales reps with highest sales"

You will likely get a bar chart but it will include all of the sales reps but it will be sorted by those with the highest sales.