Going back to previous employer

[suee]

I am on h1b changed (transfered my h1b visa) from employer A to employer B.
After 4 months i am returning to my previous (A) employer. They did not revoke my original h1b visa!
Am i allowed to go back and resume work with them without any additional paperwork? I mean is my original h1b still valid?
Or does the employer have to file another I-129 h1b application?
Thank you!