The Verdict: Not eligible
The fourth-generation status closes at 35, so this route is not open to you. Look at our Skilled Migration and Golden Visa routes for Japan instead, unless a recount makes you third generation.
The age limit of 18 to 35 is measured on entering Japan and has no discretion. No other provision covers fourth-generation descendants.
Count again from the ancestor born Japanese. If your grandparent was born as the child of a Japanese national, you are third generation, and the ordinary Long-Term Resident route has no age limit, no cap and no language test. The emigrant ancestor's family register (koseki) shows which you are.