Yikes, that's an even worse drop off than I imagined.
Not to say an engineer at 4 years isn't "experienced," but given that the pass rate halves between 4 years and 10 years of engineering experience, do you (in your own "not officially endorsed by NCEES" opinion) feel that the test is perhaps a poor reflection of mastery of engineering?
Or do you feel it's a symptom of the selection bias of the population of more experienced engineers taking it. As in, they're either more likely to not be doing technical engineering work, and/or they are attempting to get a PE as a "resume bullet point" and are maybe light on actually using it at the time? Just throwing out some possible cases as I see it as talking points, but I'd personally be a bit alarmed at that result if I were creating a test to show "sufficient mastery of practical engineering skillset and knowledge."