I'd echo the change all 4 advice. I had this come up about 3 months ago, didn't notice any difference to the cold start or running. Tested the plugs and one was faulty so changed it, 5 days later a second one failed so changed it, about two weeks later another one failed so changed both the remaining plugs, all OK since.
Dead easy to change if you've a deep 9mm socket and when I tested them it was very obvious which was faulty. No need to remove them just take the connector off and measure the electrical resistance between the end of the plug terminal and the engine. Good one measures less than an ohm when cold, bad one was anything between 500 ohm and open circuit.
I guess a dealer would probably quote an hours labour but I reckon it took me about 30 min to do all four.