Since Forgus got married in 1840 it is possible that the older unknown female is Mariah Jenkins, his wife. David and Lewis are listed in separate households in 1840. Nancy did not get married until 1845 and David did not get married until 1842 so the unknown male and female are not their spouses. The younger unknown male child is probably a relative or a friend living with the family at the time of the census. The younger unknown female may never be identified.
More research needs to be done to resolve the unknown people.
Alfred married Kizzah Jenkins, the sister of Mariah. Self listed in dwelling 66 in Thompson Township living with the Ephriam M. Orrick family. To which are added Texas Citizenship Lists, 1821-1845 and other early records of the Republic of Texas by Marion Day Mullins there is a listing for a Locky Self, farmer. He is shown to be between forty and fifty years old. Lewis is listed in Missouri Township in the 1840 Pike County AR Census.