H1b- H4 COS withdrawal

Hi ,

My current job with employer A is ending on Dec 31st and planning to submit my COS to H4 immediately.But I forsee a job offer with another employer B in Jan.Employer B won’t transfer h1b.But my employer A do not want to wait till Jan.So plan to go ahead with the COS to H4.After submitting COS to h4 and then got the offer in Jan , what are my options to start working again.My current employer will be happy to take me back on h1b if I am confirmed the offer in Jan.

Please let me know the process of moving back to H1b after COS to h4 is send to USCIS ?