Can I take H1b transfer from my current employer to a new employer if my H1b extension is still under approval process?

have my h1b extension filed in Nov 2016 based on the I140 that was approved in the same month. However, I have not received the approval yet and based on USCIS timelines, it may take another 3-4 months in regular processing. I wish to change my employer due to better opportunity elsewhere. I have 4 payslips that have been generated so far. so, my ques is: 1. what all docs are required to file for transfer of H1b? 2. Can I do it even though I have H1b approval still in works , based on my i140 approval? 3. Is my current employer legally bound to give me i140 approval copy? What are my options to transfer my H1b