Not sure why a datepicker wouldn't work. If you want help there you need to add more information on your problem.
Regarding if 4.0 is stable: It depends on hosting. 4.0 is not stable at all for azure web apps because of recycling errors. We are still waiting for a response from nopcommerce. They are currently ignoring the error it seems. It should have been a hotfix for 4.0 instantly