Using an already existing H4 visa

[balurp]

My wife is currently in H1 visa and working in US. But, she also has a H4 stamping valid till 2017 in her passport (she was in H4 earlier). Now, I've an approved I-140 and trying to apply H4 EAD for her. So, to convert her H1 to H4, will the below option work?
1) Go out of country say to Canada or India and then come back to US using the valid H4 stamping in passport and then apply for EAD? (if it's possible) 

Since we converted from H4 to H1, would the H4 stamping on her passport be considered valid if we try to use it to enter US again?

[jairichi]

Yes, she can exit and enter US with the valid H4 visa and a copy of your I797. Entry to Canada needs a visitor visa.

[rahul412]

Yes that's the fastest way to get H4.