Season 4 is on DVD now. It's been out for a few months now.
Supernatural season 4 was released September 1, 2009. You can find it at Target, Wal-Mart, Blockbuster online and other stores that sell or rent DVDs.
September 10th
Supernatural - 2005 Slumber Party 9-4 was released on: USA: 29 October 2013
Supernatural Science - 1999 Animal Telepathy 2-4 was released on: USA: 2000
September 1 2009 (in Canada/u.s)
Supernatural Science - 1999 Spontaneous Human Combustion 1-4 was released on: USA: 1999
It's Supernatural - 2007 David Jones was released on: USA: 4 August 2008
Supernatural - 2005 The Purge 9-13 was released on: USA: 4 February 2014
It's Supernatural - 2007 Steve Hill was released on: USA: 4 March 2013
It's Supernatural - 2007 Ron Phillips was released on: USA: 4 January 2010
Supernatural - 2005 thinman 9-15 was released on: USA: 4 March 2014
Supernatural - 2005 Bitten 8-4 was released on: USA: 24 October 2012 Japan: 11 September 2013 (DVD premiere)