I have moved your question to a new topic. (Please read forum rules with regard to threads older then 90 days)
Merc no longer makes Thunderbolt IV, so any replacement module is a supersession of that module. It would now be a Thunderbolt V module. But in order to tell you which one, we need to know a bit more about your engine (not the boat, we don't care what boat the engine is in)... Please provide engine model, year and serial number.
How are your wires, cap, rotor and plugs? If the energy that gets released from the coil doesn't have an easy path to ground, it finds one within the coil itself and destroys itself.
