One of the rumblings going on since Google announced Penguin 4.0 was that it might be easier to do negative SEO and/or try more link spam because Penguin now devalues web spam and doesn't demote it. Sha Menz said at SMX East last week that real time Penguin lets you experiment faster and because of that, it might make testing these spam tactics easier.
Gary Illyes from Google asked for examples of sites that they've seen hurt by negative SEO. He said he still hasn't seen any examples of this himself. Thus he is not concerned that Penguin 4.0 can be manipulated for negative SEO purposes.
Personally, I am less worried about negative SEO with Penguin 4.0 versus more link generation ideas and spam techniques to manipulate the rankings. But I assume Google can handle all of this?
Forum discussion at Twitter.
This post was scheduled to go live today but was written earlier - I am currently offline today.