I agree you have to be careful. However the takehomes I have had were a few hours of work paid at 50% of the hiring rate and that was where it ended. Great experience. There are companies out there doing this to make a great interview process and companies out there who want free work. It should become obvious which is which very quickly.
How could I be mad if you are guaranteeing pay for this moonlighting? At that time it's freelance work. However, if you're going to limit to 4 hours: If I don't have doc building copy, and tests done on that 4 hour mark, you're not getting those extras.
