Just wondering if people with i797C for H4visa can apply for EAD or should they wait until the application process is done.

I have a question wondering if you could clarify. We have an approved i140 and my husbands h1b is expiring this September and we have applied for h1 & h4 extension, I received my i797C receipt too. I see that the EAD would be valid only until the validity of my h4 which as of now is until September 2015. I was wondering if I can still apply for h4ead while my h4 extension is under processing with approved receipt number or should I wait until a decision is made on the extension. In that case is my approved i140 not of any help? Please clarify..
Thanks a lot for your suggestion. Appreciate your time and input...