--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/fj/data/fingertrees/Four.java	Fri Mar 20 21:04:03 2015 +0900
@@ -0,0 +1,39 @@
+package fj.data.fingertrees;
+
+import fj.data.vector.V4;
+import fj.F;
+
+/**
+ * A four-element prefix or suffix of a finger tree.
+ */
+public final class Four<V, A> extends Digit<V, A> {
+  private final V4<A> as;
+
+  Four(final Measured<V, A> m, final V4<A> as) {
+    super(m);
+    this.as = as;
+  }
+
+  public <B> B foldRight(final F<A, F<B, B>> aff, final B z) {
+    return aff.f(as._1()).f(aff.f(as._2()).f(aff.f(as._3()).f(aff.f(as._4()).f(z))));
+  }
+
+  public <B> B foldLeft(final F<B, F<A, B>> bff, final B z) {
+    return as.toStream().foldLeft(bff, z);
+  }
+
+  @Override public <B> B match(
+      final F<One<V, A>, B> one, final F<Two<V, A>, B> two, final F<Three<V, A>, B> three,
+      final F<Four<V, A>, B> four) {
+    return four.f(this);
+  }
+
+  /**
+   * Returns the elements of this digit as a vector.
+   *
+   * @return the elements of this digit as a vector.
+   */
+  public V4<A> values() {
+    return as;
+  }
+}