i would at least make sure the plug tip is the same all around so you dont get hot spots in one of the cyl. ie all platniums, coppers, etc. the +2 or +4 botch has isnt really that different on a normal car. you may not notice the difference at low rpm but durring WOT (wide open throttle) one of the plugs may fire more rapidly/hotter than the others...im not sure if its that bad but worse case senario is you get engine knock so i would stick with all the same plugs and all the same heat range.