Fourth … fifth: see the comments on verses 2 and 3 above.
The son of: as in the previous verse, note that Good News Translation has “whose mother was….”
Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on the First and Second Books of Samuel, Volume 2. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here .