>We have a 500 page employee handbook (it is given to us every year and we must read and sign it) that specifies that we are hired 'at will' and can be terminated 'at will' without cause. Surprisingly they expect a 3-4 week notice when an employee quits.

This surely violates federal or state employment laws.

In BC, you get 1 week pay for every year of service, plus you can challenge a wrongful dismissal.
http://www.labour.gov.bc.ca/esb/facshts/termination.htm