THE PROBLEM IS PROBABLY THAT YOUR No 4 CYLINDERS CARBURATOR MIGHT HAVE A VERY HIGH FUEL LEVEL (YOU SAY THAT THE PLUG COMES OUT BLACKENED AND WET). MY ADVICE IS TO CHECK ALL THE FOUR CARBURATORS. GET THEM CLEANED AND CHECK THE FUEL LEVEL. AS YOUR FZR IS AN ´87 MODEL, IT ´LL PROBABLY HAVE WEAR CARBURATOR NEEDLES SLIDES AND JETS. IF SO. YOU HAVE TO RENEW THEM, OR YOU´LL SOON HAVE THE SAME PROBLEM WITH THE OTHER CYLINDRS TOO.