(defn max-4-matches
  [s]
  (letfn [(max-4-matches*
       [s c]
       (cond (empty? s) c
         (or (= (nth s 0) \o)
             (= (nth s 0) \x))
         (if (= c 3)
           (+ c 1)
           #(max-4-matches* (next s) (+ c 1)))
         :else
         #(max-4-matches* (next s) 0)))]
    (trampoline max-4-matches* s 0)))