Go for 2008
Younger=Better(most of the time)
Moreover,the 04 had it's rear end rebuilt which means(as unr3al said) it's been involved in a major accident. Also, Crown Vics are prone to catching fire when rear-ended due to the fuel tanks being in the back. It's there in wikipedia. Make sure to get the carbon plate or whatever it is to protect the gas tank. Moreover,the 2008 is much cheaper which means you can spend the extra 500 bucks on something else,like new exhaust,or probably a supercharger
Also,put some HG2 Runners on it xD