I have a comparable old PR4 site that I started to work on again last month. It achieved very nice rankings without any notable backlinks. Yesterday I created 3 bookmarks and maybe 2 backlinks, furthermore I wrote one new post. After that, my site has gone - doesn't even rank for its fantasy name which has 0% competition of course.
Now the first thing I did was: Making the new post password protected and changing the anchor text of all the new created backlinks. If that won't make the site come back again I'll delete the new links I've build. I must somehow have triggered a filter at Google but can't figure out which or why. This is definitely NOT a Google Dance...
Will keep you guys updated!