My 15yr old briggs & Stratton lawn mower has the original plug. Usually starts 1 pull and never misses a beat. A scored cylinder could be a bad ring, gas washing or debris. 120K in 4yrs is 30K a year. That's 6 oil changes per year(not likely). My guess it wasn't changed every 5,000mi.It is very hard on the coils and that's what makes a lot of them fail before they would but 30,000 is probably being very proactive about it. With modern ignition systems from the early 90s on you are fairly safe to go at least 50 or 60 thousand miles.
This does depend on the type of plug but is anybody actually using plain old copper plugs anymore?? I had on occasion but you want to change them by every 30000 because the Gap Lee Road quickly. On a standard Motorcraft or Autolite single Platinum you can go 50 or 60 thousand and probably only a road the gap 5-8 thousands.
A double Platinum is going to wear even less. Irradiance would last even longer but I don't pay the money for those. There's also a trade-off on actual spark performance. Every time you get the harder materials that last longer you get less actual spark or you get a pinpoint spark that isn't exactly the best thing to promote the best combustion but in reality it's all kind of a moot point.
Nowhere ever did anybody decide that it was a really good idea to leave plugs in a hundred thousand miles... This was simply done so the manufacturers could claim you didn't have to do a tune-up for a hundred thousand miles. Buy our new high-tech engine in our new car and you will save money on maintenance and repairs.
Just like the new Briggs & Stratton push mower engines that say never needs oil changes just check and top off. If that's not a bad idea. Nothing is. All marketing.