four*.el

(defun tmp:four* (&rest l)
  (when (< (apply #'min l) 0)
    (error "All elements must be positive"))
  (string-to-number
   (apply
    #'concat
    (nreverse
     (sort (mapcar #'number-to-string l)
           #'string-lessp)))))

four.el

(defun tmp:four (&rest l)
  "Returns the largest number obtainable with subsequences L.

Write a function that given a list of non negative integers,
arranges them such that they form the largest possible number.
For example, given [50, 2, 1, 9], the largest formed number is
95021.

<https://blog.svpino.com/2015/05/07/five-programming-problems-every-software-engineer-should-be-able-to-solve-in-less-than-1-hour>"
  (when (< (apply #'min l) 0)
    (error "All elements must be positive"))
  (string-to-number
   (apply
    #'concat
    (sort (mapcar #'number-to-string l)
          (lambda (a b) (not (string< a b)))))))

(tmp:four 50 2 1 9)
; => 95021