K-4 visas are available to the unmarried children under the age of 21 of the spouses of U.S. citizens.  The visas allow the child to accompany their non-citizen parent to the United States to reunite with their U.S.-citizen step-parent.  The regulations require that the step-parent file a Form I-130, Petition for Alien Relative, classifying the K-4 beneficiary as their step-child, before the child can apply for permanent residence.  Unfortunately, the immigration law only classifies a step-child as a child if his parent married his step-parent before his 18th birthday.  That means that a child older than 18 but under 21 at the time of his parent's marriage to a U.S. citizen would qualify for a K-4 visa, but once in the United States, would not be eligible for permanent residence.  The Third Circuit invalidated the regulation requiring the filing of an I-130, finding it incompatible with the K-4 statutory provisions.

The full text of Cen v. Attorney General can be found here: