Do children of the second wife inherit as much as children of the first wife?

Yes, all the children of one father have the same inheritance rights, regardless of who their mother is. Faraid looks at the child's relationship to the father, not the order of the marriages. A son of the fourth wife receives the same share as a son of the first wife, and likewise a daughter. The condition is that the child's lineage to the father is legitimate, that is, born within a valid marriage. To make matters easier, polygynous marriages must be registered and the birth certificate must record the father's name. The father is also encouraged to be fair in lifetime gifts to all his children. See children in polygyny.