I'd gather your list of 4 PR 5 links and 10 PR 3 links before you start. I'd also install a hit counter on each page so you can measure traffic effects. Granted traffic will be relative based on the pages containing the links, but it would still be an interesting number.
Thanks Tim, no rush as I'm out of the office until Tuesday, I should be okay for URLs to test this with - though thanks for the offer. I'll be sure to post the results here as soon as I have anything conclusive.
Can you also clarify 'higher up' and in what terms you're talking about 'worth' as I'm not sure I follow you exactly.
I always go quality over quantity. In my experience, it seems more difficult to link to PR5 pages than PR3...so I would take the PR5 pages if I can.
I'll chime in on the "quality" side of the argument. PR 5 links are much harder to come by than PR 3. And I have no solid evidence on this, but it seems to me from my own experience that PR 5+ links carry a bit more weight with G than the math would indicate.
If all the PR5 and the PR3 websites are relevant and about the same quality, I'd definitely go for the 10 PR3 links. You'll not only have the possibility that you end up with 10 PR4 links, but you might also get 2,5x the traffic of 4 PR5 links (assumed that the traffic of all the websites is about the same).
I've been thinking about this more. Because you said which should you invest your time in first, I would actually go by their Alexa traffic rank. Even tough Alexa is a poor indicator of a site's actual traffic because the Alexa tool bar is usually only istalled by webmasters, if your site has lots of good link bait, webmasters are exactly the types of people you want visiting. They in turn may add natural links to your site, which is what you really want. It'd still be interesting to see which site does better for your search term, but the ultimate goal is traffic, so concentrate on those sites that get good traffic.