Here are some reasons why 4D Client and 4D Server should not be run on the same machine:

1. The main reason is that both applications will run much slower.
2. You can cause problems for the other clients.
3. You're at a higher risk of damaging your data if the client on the server machine crashes.
4. Holding down the mouse button on the server computer freezes all other clients.
5. It is only intended for testing and diagnostic purposes, not for production deployment.