filter-file.clj

(defn filter-file [filename] 
  (with-open [rdr (io/reader filename)] 
    (reduce (fn [words line]
              (->> (str/split line #"\s+")
                   (filter #(and (<= (count %) 9) 
                                 (>= (count %) 4)))
                   (set)
                   (set/union words)))
            #{}
            (line-seq rdr))))