Google's not going to "ban" your site period. You are not going to get your site banned for incoming links... think about it? If you could get banned for incoming links, wouldn't you just go and spam your competitors links to get them banned?
The worst that will happen is Google will not count the links. But that probably won't happen, because digg is democratic. If your comments get caught and burried, they won't show on the page anymore.
Which is the beauty of this method. If you find articles with PR, and <50 comments, that are over 150 days old, chances are no one is going to find your comment and bury them.
And you don't need a ton of PR4 links to pass your PR4 on to your blog (esp. if there aren't any other outbound links on the page). I would bet with about 10 or so PR4 incoming links to your site from digg, you'll find your site to be a PR4 within 90 days.
So in other words, don't go crazy spamming every page with PR. You can get a 2 or 3 PR4 link everyday for the next week, and you'll be good to go
