(define (convert-to-base-four num)
  (local
    ((define (convert lst x)
       (cond [(> 4 x) (append (cons x '()) lst)]
             [else (convert (append (cons (modulo x 4) '()) lst) (/ (- x (modulo x 4)) 4))]))
     )
    (convert '() num)
  )
  )