I do not know why the cover was charged 4 cents due. I assumed it was a double-rate penalty for unpaid mail. A google query tells me "The Penalty Policy: Under U.S. postal laws of that era, if a letter was mailed with partial postage, the recipient was only charged the exact missing amount. However, if a letter was dropped into the mail stream completely unstamped (unpaid), it was flagged at the delivery office and penalized by charging the recipient double the standard rate." But in none of the foot notes could I find the exact rules to confirm that theory, or the time period(s) where the 2x underpayment penalty was in effect to require 4c due rather than 2c.
A Linn's article on "Circumstances determined handling, fees for U.S. shortpaid mail" says if a letter was mailed without postage, the mailed item was to be returned to the sender, but "Finally, before that date [1928?],
if an unpaid first-class letter inadvertently reached the delivery office, twice the postage was due, to be collected from the addressee."
https://www.linns.com/news/us-stamp...r-us-sh.htmlIt could also be a "top of pile" example, like John Becker recently mentioned in the 1/2 cent postage due thread, where there could have been two 2-cent due letters received the same day for the same "Prussian Remedy Company" address, and the postal clerk put 4 cents of postage due stamps on the top letter rather than 2 cents on each of the two unpaid letters.