my quad is spitting/sputtering and the occasional backfiring...I change the gas and its still doing it....any suggestions?

 

Possible that the intake manifold may have a crack in it like the 700s were bad about,but the worst thing I think about is the Ducati ignition system failing,"IF" your 2004 model was an early build (before Feb/2003) that had the Ducati system. Common problem on the 700 and 600.Some of the symptoms were hard starting,back firing,erratic idle,etc. Replacement Kokusan kit is #2202603 for the 600. You can check this forum or just google Polaris 600/700 ignition problems and you will see a lot of problems.Here's on kit on ebay. Consists of flywheel and stator,cdi,coil,regulator,necessary wiring jumpers.
http://www.ebay.com/itm/New-OEM-Polaris-Ignition-Kit-Assembly-03-04-2003-2004-Sportsman-600-2202603-/201048157029?hash=item2ecf676b65&vxp=mtrPlus here's a good video on the kit installation on a 700,same as on the 600.