Four years seems a short time between magneto removals for inspection regardless of hours in use especially considering each engine has two of them.Surely the chance of both failing on the same flight is very low.
They are pretty robust. When I purchased my pride and joy with a normally aspirated IO 520 engine somehow the magnetos were placed on the life of engine list instead of the component list for periodic maintence. Six years and 1,500 hours later the engine gradually became a bitch to start especially in damp weather. Once it was going it was ops normal. It was sort of self grounding as eventually I guess it would not have started at all. As I was concerned about the starting difficulty I took it to a maintence shop and caused a bit of a flurry when the error was discovered. The overhaul was done, magnetos put on the correct list and I was on my way.
Re the SIDS currently compulsory in Australia it will be interesting in ten years or so to compare our accident rate due to lack of maintence failures with countries such as the US where SIDS is not compulsory.