function* generatorFunction() {
  yield 1;
  yield* function* () {
    yield 2;
    yield 3;
  }();
  yield 4;
}

const gen = generatorFunction();
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);