Result:
(1 (2 (3 4)))

Box-and-pointer diagram:
[.|.] -> [.|.] -> nil
 V        V
 1       [.|.] -> [.|.] -> nil
          V        V
          2       [.|.] -> [.|.] -> nil
                   V        V
                   3        4

Tree structure:
(1 (2 (3 4)))
 /         \
1          (2 (3 4))
          /        \
         2         (3 4)
                   /   \
                  3     4