H4 travel to US while H1B extension pending

Please let me know your valuable inputs on this.My family is in my home country now. Our H1 and H4 are stamped upto June-2018.We already submitted H1B extension petition. By mistake we submitted I-539 also for H4 extension.My questions are:1. Can my family travel to US in their existing H4 (valid upto June-2018) in April-2018 and go back to home country in May-2018? Will they face any issue in PoE due to visa expiry in Jun-2018?2. How can I withdraw the I-539, since I am not supposed to submit it while dependents are outside US. Can my dependents send a letter to USCIS signed by them? Do I need to sign that?Thanks in advance.