After the third round, five more 4clojure.com solutions:

(ns com.icyrock.clojure.a4clojure_com.core
  (:use clojure.test))

;;; Problem 40 - Interpose a Seq
(deftest test-problem-40
  (let [v1 #(rest (interleave (repeat %) %2))
        __ v1]
    (is (= (__ 0 [1 2 3]) [1 0 2 0 3]))
    (is (= (apply str (__ ", " ["one" "two" "three"])) "one, two, three"))
    (is (= (__ :z [:a :b :c :d]) [:a :z :b :z :c :z :d]))))

;;; Problem 39 - Interleave Two Seqs
(deftest test-problem-39
  (let [v1 #(mapcat list % %2)
        __ v1]
    (is (= (__ [1 2 3] [:a :b :c]) '(1 :a 2 :b 3 :c)))
    (is (= (__ [1 2] [3 4 5 6]) '(1 3 2 4)))
    (is (= (__ [1 2 3 4] [5]) [1 5]))
    (is (= (__ [30 20] [25 15]) [30 25 20 15]))))

;;; Problem 38 - Maximum value
(deftest test-problem-38
  (let [v1 #(first (sort > %&))
        __ v1]
    (is (= (__ 1 8 3 4) 8))
    (is (= (__ 30 20) 30))
    (is (= (__ 45 67 11) 67))))

;;; Problem 37 - Regular Expressions
(deftest test-problem-37
  (let [v1 "ABC"
        __ v1]
    (is (= __ (apply str (re-seq #"[A-Z]+" "bA1B3Ce "))))))

;;; Problem 36 - Let it Be
(deftest test-problem-36
  (do
    (is (= 10 (let [x 7 y 3 z 1] (+ x y))))
    (is (= 4 (let [x 7 y 3 z 1] (+ y z))))
    (is (= 1 (let [x 7 y 3 z 1] z)))))

(run-tests)