The scenes of Mickey Borden (John Garfield) in flashback were not clips lifted from the first film of the series, Four Daughters (1938), but rather were new footage shot for this film.
Although Ann is at least seven months pregnant when she went into pre-term labour, she could show no physical indication that she is expecting due to the Hays Code.
Dr. Forest, Jr. quits his practice to study pneumoconiosis, and be the doctor at the local mill for $40/wk (equal to $690/wk in 2016). Pneumoconiosis is an occupational lung disease caused by the inhalation of dust. What type of dust would then provide a sub-diagnosis, such as coal, asbestos, cotton, etc.
While some sources claim that Four Wives (1939) was shown at some engagements with Warner Bros.' new Vitasound audio process, a non-stereophonic process that combined a standard, variable width monophonic soundtrack with a second, variable width control track, located between the soundtrack and the sprocket holes, that increased loudness for certain scenes by switching on additional amplifiers and speakers, only a segment of the film was actually reproduced in Vitasound as part of a demonstration of that sound system that took place in late November 1940, nearly a year after the film's opening. Santa Fe Trail (1940), released in December 1940, was the only film fully presented at some theaters in the Vitasound process.