[1] (let ((x (list nil))) (car x)) nil
[2] (let ((x (list nil))) (setf (car x) 5) (car x)) 5
[3] (flet ((g () 5)) (g)) 5
[4] (flet ((g () 5)) (setf #'g #'(lambda () 7)) (g)) error
The error is "FUNCTION not a SETF-able form." or words to that effect.
If in 4 you meant to setf (symbol-function 'g), then the answer is 5.
-- Scott