I bought some new CR8E NGK plugs recently, one pegged out at 8 weeks, then another one or more at 10 weeks.
Noticed at first that pipe #4 wasn't hot, plus the bike was running horribly, but when i changed that plug for an old one it ran okay again for a couple of weeks, then it happened again so put all my old plugs back in and it all ran fine again.
Bought new Iridium plugs on Monday and now everything is as it should be.
Not really ever had plug trouble before in years of riding but that's what mine turned out to be so worth a thought.