Write to SONY at SPHE_Customer_Service@ spe.sony.com to let them know you want season 4 on DVD. My e-mail to them garnered the following response:
Thank you for your e-mail and for your feedback! We appreciate your
interest in Sony Pictures Home Entertainment (SPHE) products. The
information you provided has been forwarded to the appropriate department for
research and review. Unfortunately, we currently do not have a published
release date for this title.
So at least they're keeping track of how many people write in to request it.