The indicia says "Vol. 1, No 5", but the cover has "Number 4" on it.
On sale date from the publication date found in the Catalog of Copyright Entries, Part 2, Periodicals, New Series, Volume 35, 1940, Number 2. Second class permit. Copyright number 449976. Issue number 5 was the number reported to the Copyright Office.
This issue contains a C. C. Beck illustrated advertisement (1/4 page) for the Captain Marvel Power Siren, which could be purchased at newsstands or obtained free if the reader cut out and mailed in three certificate coupons that appeared in Fawcett's Whiz Comics, Master Comics and Slam-Bang Comics. This ad is reprinted in Alter Ego #141 (August 2016), page 79.
Captain Marvel also wears button down flap in this issue on splash page, pages 10 and 12.
The last 1/4 page of the story is an illustated ad, with Captain Marvel, for the Captain Marvel Power Siren (the World's Mightest Whistle).
Next Issue: Rollo mystery.