Short plugs in the JP4 will cause very poor flame propagation because of the recessed spark and the exposed threads in the hole will soot up making it difficult to fir the right plugs later, but at least the threads could be cleaned out.
Long plugs in a J4, besides the possible damage, the plug's exposed threads will foul with soot and you'd be at great risk of damaging the head winding the sooty plugs out of the head.
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