H4 Filing in case passport about expiring

H1B transferred and approved to new employer and H4 is still with previous employer.

H4 didn’t filed along with H1B transfer. I have resigned from previous employer.

Dependent passport not valid more than 6 months, so new employer asked to get renewed passport for applying H4 transfer (i539) to new employer. Renewal of passport takes minimum 2-3 months.

If previous employer revokes my H1B before applying H4 I539 with new employer, what is going to happen to H4 (which with previous employer still) ?

How soon H4 (I539) should be filed with new employer after getting renewed passport ?

Please advise.

You can apply for tatkal and get it approved in few days.