According to Wikipedia, although starting with Emperor Sujin, they are with a direct possibility of existence, they still presumed legendary until Empress Jingu. However, Kofun Culture (http://www.t-net.ne.jp/~keally/kofun.html) reference webpage said emperors starting with Emperor Keiko, they might be real (until Emperor Chuai). This webpage already corrected the reign time of Emperor Ojin as to 370 - 390, but the others provides only century period. Emperors after Emperor Nintoku (not including him) appear to be aligned with the actual reign century period mentioned there. What is the exact reign time year periods for these emperors?
Please note that the non-research data below comes from Wikipedia. I think the reign time of Empress Jingu should be 361 - 370, because late xth century generally refers to the 61th-100th or 71th-100th year, the first one is ok, and the second one is duplicated with reign time of Emperor Ojin. Emperor Nintoku's reign time should be 390 - 400, which is the blank of Ojin (r. 370 - 390) and Richu (r. 400 - 406). Because Emperor Seimu reigned around the first half of the 4th century, the reign time of Emperor Chuai should be 351 - 361 (which the year 351 is the start of the second half of the 4th century). Wikipedia said that both Keiko and Seimu reigned around the 4th century and the second one is recorded as the "first half of the 4th century" in Wikipedia. Due to lack of century period (early/mid/late), I can get reign time conclusion of neither Keiko nor Seimu.