This was one of the questions on chat with Att. Murthy (My attorney also gave me the same answer a couple days back).
Chat User : A spouse on H-4 wants to use his / her EAD while I-140 is pending. Will s/he be out of status if the I-140 gets denied and can s/he come back to H-4 again?
Attorney Murthy : As long as the H-4 status is valid, a person may maintain both the H-4 and the I-485 pending status. If the I-140 gets denied, the person will be eligible to revert to the H-4 status as long as the I-94 card has not yet expired. If the H-4 status is about to expire, the person should either file an extension or, upon the I-140 denial, should plan on traveling abroad and applying for the H-4 visa stamp at the consulate and reentering on H-4 status in order to live legally in the U.S. In such cases, it may require the person who was not on any status even for a single day to mention that s/he was "unlawfully present" in the U.S. on the visa application form at the consulate. The dual intent doctrine under law allows one to change between the H status and the I-485 pending status, and both may be valid simultaneously, so it is safest to maintain both whenever possible. These issues have been crystallized in the March 2000 and May 2000 Legacy INS Memos and may be reviewed on MurthyDotCom.