By Stanley White and Vidya Ranganathan TOKYO, May 20 (Reuters) - Bitcoin recovered marginally on Thursday from the previous session's brutal slide to four-month lows but was weighed down by concerns over tighter regulation in China and unease over massive leveraged positions in the cryptocurrency world. Bitcoin, the biggest and most popular cryptocurrency <BTC=BTSP>, rose 8.75% to touch $40,000, albeit briefly, after plunging 14% on Wednesday to its lowest since late January.