fourth son ... fifth son ... the sixth

This is the numerical order of David's sons. (See: rc://en/ta/man/translate/translate-numbers)

Adonijah ... Shephatiah ... Ithream

These are the names of David's sons. (See: rc://en/ta/man/translate/translate-names)

These are names of David's wives. (See: rc://en/ta/man/translate/translate-names)