Question: Who is Alexander Stewart, 4Th High Steward Of Scotland's paternal grandfather?
Answer: Alan fitz Walter

Walter Steward of Dundonald (died 1246) was 3rd hereditary High Steward of Scotland and Justiciar of Scotia. He was the eldest son of Alan fitz Walter, 2nd High Steward of Scotland but which wife of Alan's was his mother is not clear. He was the first to use Stewart as a surname, and was designated "of Dundonald"