V3 extends @Nullable Object,
          V4 extends @Nullable Object>
      Combiner4<V1, V2, V3, V4> whenAllSucceed(
          ClosingFuture<V1> future1,
          ClosingFuture<V2> future2,
          ClosingFuture<V3> future3,
          ClosingFuture<V4> future4) {
    return new Combiner4<>(future1, future2, future3, future4);
  }

  /**