Series IV. Related Family Papers contains material from other Bowditch family members or individuals related to the Bowditch family by marriage.

Subseries A. Early Ancestors' Papers contains a musical score by William Bowditch, copies of wills of William, Ebenezer, and Habakkuk Bowditch, and an account book/journal by an unidentified man. The author of this journal stayed with Mr. John Coburn of Boston while recovering from an illness. He was treated by Dr. Nathaniel Perkins and was visited by his brother, James.

Subseries B. Nathaniel I. Bowditch Papers contains notebooks created by Nathaniel's son, Nathaniel Ingersoll Bowditch (1805-1861) and two letters.

Subseries C. Babcock Family Papers contains the correspondence of Babcock family members who are related through Nathaniel Ingersoll. It also contains typescript copies of the journals of Rev. William Smyth Babcock (1764-1809), a Freewill Baptist minister preaching in Vermont and New Hampshire.

Subseries D. Swann Family Papers contains letters, calling cards, and a will from the Swann family to which Margaret Swann Bowditch, wife of Ernest William Bowditch, is related.

Subseries E. Lyon Family Papers contain correspondence of Lizzie and Henry Lyon, ancestors of Margaret Swann Bowditch.

Subseries F. Ingersoll Family Papers contain correspondence, financial records, a diploma, and genealogical charts for the Ingersoll family, who are related to the Bowditch family through Mary Ingersoll Bowditch (1781-1834).

Subseries G. Harold Bowditch Papers contains correspondence, clippings, genealogy, printed materials, and a radio playlet about Nathaniel Bowditch compiled by his great-grandson, Dr. Harold Bowditch. Subseries H. Other Bowditch Papers contains four letters, one each from J. Ingersoll Bowditch, Henry I. Bowditch, Charles P. Bowditch, and Vincent Y. Bowditch.