Copyright information was found in the Library of Congress online archive for this issue. The copyright date is listed as September 8th 1944. The cover date is listed as November 1944.
Based on copyright dates, Grim Reaper’s appearance in AMERICA’S BIGGEST COMICS BOOK #1 was published between his appearances in WONDER COMICS #3 and #4.
This is part 4 of a four-part story arc.