The Verdict: Need More Info
You are too young for the fourth-generation status, but it opens at 18. Use the time to learn Japanese and gather the family documents.
Fourth-generation descendants have a separate Designated Activities status, capped at 4,000 people a year and open to ages 18 to 35 at entry. Applicants aged 18 to 30 need Japanese at JLPT N5 (or proven N4 ability); those aged 31 to 35 need N3. You also need a supporter living in Japan for at least your first three years, and proof of descent through the Japanese family register and every generation since.
The stay runs in short renewals to a five-year maximum. After that you can change to Long-Term Resident status, where settlement really starts, but only with stronger Japanese, good conduct and independent means.
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.