All my testing points to the fact that they are a little better, but much of the time it makes very little difference...
The bigger question being, can you get more links to stick by using varied content, rather than sticking to one genre!
I've found that by using massively varied topics, I'm able to produce considerably more volume of original content and links, and this therefore completely outweighs any benefit from 'relevant themed' sources.
Also; the algorithms to parse content into themes/memes are incredibly complex; and although it's easy to believe that Google has unlimited power; they don't have the computational power to properly LSI even 1% of the web, let alone all of it; so they have to use simpler methods; basically word-relevance grouping, and very simple language parsing.
This means that the majority of the time, you'll get as much juice from a 'relevant' PR4, as you will from an off-theme one. I've tracked data around this subject now for nearly 5 years across tens of thousands of domains and hundreds of thousands of keywords; as I do large scale SEO for some big customers. And I've yet to find any concrete evidence that theme-relevance makes any significant difference to the power of a link; there's always some other factor at play. Google is NOT as clever as many think.