The four modes of life form a ladder or flight of steps. That fight attaches to Brahma. By ascending that flight one reaches the region of Brahma.
चतुष्पदी हि निःश्रेणी ब्रह्मण्येषा प्रतिष्ठिता। एतामारुह्य निःश्रेणी ब्रह्मलोके महीयते॥