I e-mailed Universal about Season 4. Here's their response, which came quite quickly.
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"Thank you for contacting us. No release date has been announced for this title. Unfortunately, no further information is available. For a listing of upcoming releases, please check our website periodically at:
Well thanks for the info, Headache, or I guess, lack of. That really stinks. Here they have a show that was so terrific, with so many fans across the world, and they are dragging their feet. Ridiculous! I wonder what the sales were from the 3 seasons that have already been released? Maybe they weren't good? But I don't really understand that. There are too many people out there that love Columbo...why would it be a slow sell in the dvd sales?