(Now Jonathan, Saul’s son, had a son who was crippled in his feet. He was five years old when the news came from Jezreel about Saul and Jonathan, and his nurse took him up and fled. But it happened that as she hurried to flee that he fell and became lame. His name was Mephibosheth.) Bible

“had a son.” This event about Mephibosheth seems to be inserted to assure the reader that the house of Saul had not been wiped out by the death of Ish-bosheth. Mephibosheth was still left.

Commentary for: 2 Samuel 4:4