There are currently two Discworld novels being written as of August 2012.
The 40th book could either be called Raising Taxes or Scouting For Trolls. The release dates for either are still to be confirmed.
The 40th Day - 2013 was released on: USA: 19 October 2013
I believe its Mozart's 40th Symphony
The 40th Annual Emmy Awards - 1988 TV was released on: USA: 28 August 1988
The 40th NAACP Image Awards - 2009 TV was released on: USA: 12 February 2009
In the Life - 1992 40th Anniversary of Stonewall 17-9 was released on: USA: June 2009
The 40th Annual Daytime Emmy Awards - 2013 - TV was released on: USA: 16 June 2013
40th = fortieth.
Bob Hope Salutes the U.S.A.F. 40th Anniversary - 1987 - TV was released on: USA: 25 May 1987
The 40th Annual Academy of Country Music Awards - 2006 TV was released on: USA: 15 May 2006
Job Corps' 40th Anniversary A Celebration of Success - 2004 was released on: USA: 29 September 2004 (Washington, D.C.)
Gala Tribute AFI's 40th Anniversary - 2007 TV was released on: USA: 3 October 2007 (Hollywood, California)
The 40th Annual People's Choice Awards - 2014 TV was released on: USA: 8 January 2014 (TV premiere)