H1 transfer while on H4: when to start work

[penguin80]

Hi,
I was in US on H4 visa. My H1B was approved with counselor processing for company A. I went to my home country for H1B visa stamping but was denied and petition was sent to USCIS for review.
I came back to US on H4. I am applying for H1B transfer from company A to company B with COS. Can I start working immediately after I get receipt number or do I have to wait for COS approval to come thru?

Thanks for your help in advance.

[shervin143]

You cannot work until the H1B gets approved with a new I-94 (COS).