Bitcoin sank to a four-month low on Friday and was on course for its worst weekly performance in a year, hurt by uncertainty over whether Joe Biden will remain the Democrats' U.S. presidential candidate and by worries about potential increases in crypto supply. Bitcoin prices fell 5% to $55,366, their lowest since February end on Friday, and was down 10% for the week. Investors are also fretting about the possibility of Biden being replaced as the Democrats' presidential nominee by someone who is less pro-crypto, market participants said, after his poor performance in the first debate against Donald Trump.