When you say the plugs look good on some and fat on others is this based on cruise plug readings or on wide open throttle plug readings? If at cruise try opening the idle air bleed .003 to .006 on the bleed in the center carb. that feeds those cylinders. I would tell you which side but I don't have a 340 six pak intake to look at, sorry. To answer the main question you can run one step hotter plug to mask the problem and probally get away with it, but if the WOT is lean then your taking a chance of causing those four cylinders to go into detonation Let us know what you end up doing to solve that please