140 Portability and H4 EAD validity

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This is in reference to the new I-140 portability law.

I have a approved I-140 from company A and it was approved for more than 180 days. I found a new job with Company B which will now transfer my H1-B to its name. With the new law I can also retain my I-140 and priority date. In the meantime, my spouse has a H4 EAD based on my I-140. If I join company B, under what circumstances my spouse will lose her EAD? Company B promises to start my new green card application but it will not happen immediately.

I am asking this question as there is some more information needed on what I-140 portability actually means and how will it affect H4 EAD validity.