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.
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.