Transferring to previously approved H1B from H4

My wife had an H1B with an employer in India (Employer A) and has worked in the US (at client location: Client A; Since 2016). In 2019 she went back to India worked for Employer A for few more months and then resigned in April 2020. Employer A has pulled back the petition in September 2020 (or revoked visa; not sure if it’s the correct terminology).

She is currently working for another Employer B in India and now has an H4 visa (has traveled on same one time; Nov 2020- June 2021).

I’m trying to find answers for:

  1. Can she come back on H4 is next 3-4 months and an employer can file for her H1 (H1B transfer or a cap exempt petition/ not going through the lottery again; not 100% sure about the terminologies)?

  2. Once approved she can work and use her 3 and half years of unused H1B duration immediately (as she has already used about 2 and half years out of 6 years)? Or the start date has to be 1st October like the regular lottery applications?

  3. Is there any time line before which the above process must happen, like 3 years or 6 years from the time of visa revocation? or there is no time limit?

  4. Any further details required for conclusion on this request?

Any help or direction on same will be great.

I am assuming you are on H1B in the US and so she got H4 visa. If yes, she can travel to the US with a valid H4 visa, copy of your current I-797, employment verification letter for primary H1B , last three pay stubs for H1B and other H4 related document to present to the CBP at port of entry.
If she gets a H1B employer, the employer can file for cap-exempt H1B provided she has some time left from her previous H1B term of 6 years.

Yes. Time spent in the US while working on H1B will be counted against the 6 year term.

No time limit.