H-1B to H4 CoS

My wife, who is on an H-1B, want to change her status to H4 and get the EAD based on my approved I-140. I wanted to get clarification on the timeline and dates.

Her current H-1B is valid till Jan 2020. My H1-B is expiring in March 2018 and my employer will be filing for an extension soon. Since this will be done via premium processing, I will be filing for my wife's CoS along with my petition.

My question is, would my wife's application will also be covered under premium processing and if so, will she get her H4 and H4 EAD in 15 days (assuming no RFEs)? Also, once her status changes to H4, would her H4 be valid immediately, or will it only begin with my H1 extension (i.e., April 2018)? Any clarity you can provide would be much appreciated. Thanks!