In 1640 Saxe-Eisenach was again separated off from Saxe-Weimar for Albert , son of John II of Saxe-Weimar , but after just four years he too died without an heir , and Saxe-Eisenach was then divided between Saxe-Gotha and Saxe-Weimar , ruled by his brothers Ernst and Wilhelm . In the course of the partition of the Saxe-Weimar lands among the sons of late Duke John II in 1640 , the eldest brother William of Saxe-Weimar gave the re-established Duchy of Saxe-Eisenach to his younger brother Albert IV , while the youngest Ernest received the Duchy of Saxe-Gotha . However , Duke Albert died without heirs in 1644 , and Saxe-Eisenach was then divided between Saxe-Gotha and Saxe-Weimar , ruled by his brothers .