The 4% come from a application-level benchmark that was popular back then. This is about ARM9/armv5 too, which is not suited for any multi-user OS as address sp

The 4% come from a application-level benchmark that was popular back then. This is about ARM9/armv5 too, which is not suited for any multi-user OS as address spaces switching is very expensive compared to today's architectures (armv6+) due to the needed cache flushing on every address space switch. This has nothing to do with L4 variants, this is an architectural issue.