Validity of Stay after i94 extension for h4 Visa has been filed

[buntydash]

Hi There
My wife is expected to travel by May 27 and her H4 visa is expiring on 30th June . If I am filing an extension as soon as I get the updated i94 ,probably by 1st June and I dont get a receipt number as an acknowledgement from the USCIS before June 30 , will her stay be illegal till I get the receipt number from USCIS ? Kindly advise !