Based on design, then Bosch......

That was what I meant, the super 4 design has an open center electrode, with 4 ground electrode at the side. Spark can come from any of the 4 ground. If driving twin cam engine, plug tip is open directly in the path of the piston head, therefore getting combustion.

Denso Iridium has 0.4mm center electrode, and NGK I think is 0.6mm/ Denso claim the 0.4mm smaller sharper tip, collects or concentrate all the spark together giving a stronger spark.

The above are all claims by their respective spark plug company/distributor......

how often does one have to change spark plugs? if they haven't been changed for over ten years? safe to change them now?

I won't spend that kind of money on iridium plugs. Have used them before and actually I find them the same.
I would say the different makes of material is more for durability. Iridium being the hardest last a longtime, the platinum, then copper and so on. Platinum can be considered as they cost around RM70 (I think) and Iridium are doubled that.

Iridium may last longer, but I doubt you will get double performance. All you will get is smoother drive....

Try Denso then. Change accordingly using their conversation chart from your current model.

It is the ground electrode that wears out the plug. The spark fires from the center electrode to the ground electrode. It is the ground electrode that gets blown to bits. I have seen that happen on my single iridium and platinum plugs after 20000km in my previous car. In my current car, I wear out the NGK Double Laser platinums (which is the recommended stock plugs for my car) at about 50000km.

spare part shops don't carry them?

They should have, anyway most of the ones here have.......

It's hard to find in Sabah because Sabahan are very stingy one...

search at Garage Sales here and ask the seller to post it to you.
cheaper than wasting fuel going round and round asking 1 by 1 shop.

But rich......
or no wonder they are rich......

2 new plugs or 2 sets??

They should be able to tell you if they are Bosch dealer.
Anyway, for Bosch the heat range is reverse.
Jap plugs like NGK 5 - hot, 7 - cold, meand bigger number is cold
Bosch 6 - cold, 8 - hot, means bigger number  is hot

Oh! you don't know about Hot plugs and cold plugs also. Well certain engine are designed to run hot, so they use cold plugs. The plugs helps to cool the engine. Or if running more on highway, cold plug is used.
Town driving usually hot plugs are used to help complete combustion.
Overall just follow your car manual recommendation on the plugs.
I posted the hot and cold just for your info because you want to use Bosch and if your car is running on NGK number 5 plugs and the Bosch dealer gives you a Number 8 Bosch plugs you may be confused. So now at least you should know NGK 5 is Bosch 8....hope you understand up to here?

Looks like you still don't get it. Does not mean cold is better. Cold plugs drive in town may build up a lot of carbon if the combustion is not complete.
The number are just for guide in changing from NGK to Bosch.
For Denso they use 18, 20, 22 and so on. Smaller number is hot, bigger is cold.

Nope, just follow your car recommended plugs....change Brands, then follow their chart...

plugs won't give extra power, it will give better sparks which in turn give better combustion and helps the flows of engine operation running smoothly.

actually such information (ie. how thing works) can be easily obtained by just googling.