there is really no need for platinum electrode spark plugs for the automobile engine. and the bosch platin +4 is a most egregious joke. it is a gimmick intended to remove more dollars from your pocket than you need to have removed.
platinum found it's way into spark plug electrodes in ww2. upon the observation that platinum would resist lead salt fouling somewhat better than nickel alloy electrode spark plugs in the heavily tetraethyl lead-additive avgas blended for the compound turbocharged/supercharged aviation engines of the era.
on the other hand, platinum had more pronounced fouling[gap bridging] problems than nickel alloy electrode plugs as platinum has a peculiar affinity for attracting nodules of certain lube oil additives[zinc dithiophosphate in particular].
i worked with AC and autolite when they introduced spark plugs with electrode attachments of platinum. their thought was that these plugs would outlast nickel alloy electrode spark plugs, allowing a set of plugs to last 100,000 miles. i know that in my last meeting in flint the AC engineers admitted that the those plugs never made that mileage mark and that there was virtually a secret campaign to freely change those spark plugs whenever the dealer could get the car so that the illusion could be created that the original equipment installed plugs lasted that long.
NGK may have started this precious metal nonsense for automotive application engines. sheer press agentry. and my engineering records can prove it. my favorite story in this regards involved daimler-benz om407h engines converted to run on CNG for transit bus service in Perth, Western Oz. I became involved because the standard plug fitted by D-B[a champion L82YC] had a hard time lasting a week.
I supplied the transit bus operator special plugs using gold palladium electrodes. shortly after installing them, the transit bus operator asked me if we could furnish spark plugs with platinum electrodes like NGK's, because they started the engine so much better. though disbelieving the appraisal of the transit bus fleet operator, we fabricated plugs identical to the electrode, heat range design of the NGK plugs and then traveled to Perth to install them and check out the engines.
before we installed our plugs, we asked to have the bus outfitted with the NGK's trialed for us. what a surprise. we timed the starting cycle. with the NGK's it took over 3 minutes to start the engine. my associates and i said to ourselves, "...and this is thought to be good?"
before we put our platinum plugs in the engine, i was taken aside by the maintenance director who asked me how long our plugs would last. i said, i had no idea. then i asked him, how long were the NGK's lasting? and i received a very honest reply, "we're lucky if they make 10,000km."
we were shown used NGK plugs and they all exhibited severe electrode erosion.
we examined the engine and we found why it might have been the case that platinum electrodes were able to start the engines faster[albeit too slowly]. the engines were outfitted with propane carburetors as opposed to cng carburetors. and the fuel valve was slaved to a shut-off governed by battery amp draw-down.
long and the short of it was that the platinum electrodes finally started the engines faster because of catalytic ignition.
once the engine's fuel delivery system was remedied with a cng carburetor our gold palladium electrode spark plugs started just like spark-ignited engines should[instantly] and outlived the NGK platinum plugs by a factor of over 10:1.
interesting side note, d-b hated anyone else playing with these engines. the engine that we were allowed to work on would become the last one to run. and it eventually had to be retired because d-b had not provided enough cooling for the engines.
none of what we showed the transit bus operator was ever accepted by d-b. they never altered their incorrect carburetion system, they never altered their horribly inadequate ignition system, they never improved the cooling. they never did anything to make the engines work. this incident taught me much about ego at engine manufacturers. their motto often is, DON'T CONFUSE ME WITH REALITY.