Google doesn't favour HTML5 over HTML4 per se. However, HTML5 does allow more semantic mark-up, which will make it easier for Google to figure out what's what on any given page. This allows Google to be more precise when it comes to ranking what is and what isn't important on a page.
For example, the
nav element indicates very clearly to Google (or any bot, for that matter) that the links contained within are the "main" links for the various sections on a website.
I've never heard of a page dropping in ranking after adopting HTML5 doctype, though I have seen and heard of more than a few that have risen in ranking (mostly for branded keywords) after making the change. Though I've always (personally) attributed that to the more semantic nature of HTML5 than any preference of Google's.