From TVbytheNumbers.com:
"Unit sales and revenue were both way up for Supernatural: its fourth season DVD set sold 238,081 units and earned an estimated $8.7 million. Last year in its first week of release the season three DVD set sold 104,979 units with estimated revenue of $4.1 million."
Supernatural Season Four DVD and Blu-ray (Region 1) were released on September 1, 2009.