If you want an NPrinting report to send every 4000 increase in sales.
You will have to start the Nprinting report using the API. With this you need to make a check within you Sense reload script and make it use the API when the condition is met.
You can also use the notification connector(SMTP connector now) to send an email alert when this condition is met.
Or make Nprinting start every so often but have a condition setup that it only starts when the condition is met.
For this last one I am not sure what the exact options are through Nprinting and what needs to be done to do this. But 90% sure it can work.
The first two options I know will work.