Iridium spark plugs have worked great in every 4.0 engine I've tried (5 and counting). These spark plugs, new distributor cap with brass contacts, new plug wires and a new coil has done wonders in all of my 4.0 engines. Whenever I get a new Jeep that's the first thing I do.
What kind of vehicle do you drive? 1988 Jeep Comanche