Always all four.
Just like tires you want to match them up unless told otherwise.
And, just like tires there are times you will have front and back different, but that is on a case by case basis.
My recommendations are to appeal to the broader sense, but happy to help with custom type questions on PM's so not to get off track.
2013 S4 Loaded (and NO RFT's) Awesome is an understatement
2008 MT X3
RIP you were a faithful sidekick and will be missed.
2007 AT X3 RIPOS
2000 Jeep Cherokee
1997 Twin Turbo RX-7
1984 GTI Wolfsburg Edition Neuspeed and more