K-4 is the "child" of a USC as defined in INA 101(b) which is: (most of those commercial websites ASSUME that the K-4 is probably a stepchild) The K-4 is for bringing a child to the U.S. in order to file an I-485 for a greencard.
(b) As used in subchapters I and II of this chapter--
(1) The term ``child'' means an unmarried person under twenty-one
years of age who is--
(A) a child born in wedlock;
(B) a stepchild, whether or not born out of wedlock, provided
the child had not reached the age of eighteen years at the time the
marriage creating the status of stepchild occurred;
(C) a child legitimated under the law of the child's residence
or domicile, or under the law of the father's residence or domicile,
whether in or outside the United States, if such legitimation takes
place before the child reaches the age of eighteen years and the
child is in the legal custody of the legitimating parent or parents
at the time of such legitimation;
(D) a child born out of wedlock, by, through whom, or on whose
behalf a status, privilege, or benefit is sought by virtue of the
relationship of the child to its natural mother or to its natural
father if the father has or had a bona fide parent-child
relationship with the person;
(E)(i) a child adopted while under the age of sixteen years if
the child has been in the legal custody of, and has resided with,
the adopting parent or parents for at least two years or if the
child has been battered or subject to extreme cruelty by the
adopting parent or by a family member of the adopting parent
residing in the same household: Provided, That no natural parent of
any such adopted child shall thereafter, by virtue of such
parentage, be accorded any right, privilege, or status under this
chapter; or
(ii) subject to the same proviso as in clause (i), a child who:
(I) is a natural sibling of a child described in clause (i) or
subparagraph (F)(i); (II) was adopted by the adoptive parent or
parents of the sibling described in such clause or subparagraph; and
(III) is otherwise described in clause (i), except that the child
was adopted while under the age of 18 years; or
(F)(i) a child, under the age of sixteen at the time a petition
is filed in his behalf to accord a classification as an immediate
relative under section 1151(b) of this title, who is an orphan
because of the death or disappearance of, abandonment or desertion
by, or separation or loss from, both parents, or for whom the sole
or surviving parent is incapable of providing the proper care and
has in writing irrevocably released the child for emigration and
adoption; who has been adopted abroad by a United States citizen and
spouse jointly, or by an unmarried United States citizen at least
twenty-five years of age, who personally saw and observed the child
prior to or during the adoption proceedings; or who is coming to the
United States for adoption by a United States citizen and spouse
jointly, or by an unmarried United States citizen at least twenty-
five years of age, who have or has complied with the preadoption
requirements, if any, of the child's proposed residence; Provided,
That the Attorney General is satisfied that proper care will be
furnished the child if admitted to the United States: Provided
further, That no natural parent or prior adoptive parent of any such
child shall thereafter, by virtue of such parentage, be accorded any right, privilege, or status under this
chapter; or
(ii) subject to the same provisos as in clause (i), a child who:
(I) is a natural sibling of a child described in clause (i) or
subparagraph (E)(i); (II) has been adopted abroad, or is coming to
the United States for adoption, by the adoptive parent (or
prospective adoptive parent) or parents of the sibling described in
such clause or subparagraph; and (III) is otherwise described in
clause (i), except that the child is under the age of 18 at the time
a petition is filed in his or her behalf to accord a classification
as an immediate relative under section 1151(b) of this title.
