They obviously have some quality backlinks to have a PR4. I would concentrate on promoting your blogs in other blogs etc and anywhere you can advertise to people in your niche to increase your traffic. Write a lens on Squidoo, try get a link in Wikipedia there are loads of stuff you can do. Look around DP, loads of ideas are discussed here on a regular basis.
try to do an on-page optimization...improve your contents with your targeted keywords but avoid keyword stuffing...then improve your link popularity...use your keywords as your anchor text that might help you rank well on SEs and eventually receive traffic from them..
I would advice you to check out your sitemap account and see is there any slight traffic for targeted keywords? If no, then you may want to consider optimizing your site for different keywords as your current targeted ones are very uncommon..
I'm going to agree with KC-tan. PR has nothing to do with traffic only SERP's do. If your keywords get no searches, then a PR of 9 isn't going to help you. For instance if your keyword was Red Widget GX456. If there are no searches for that keyword phrase, you're not going to get any traffic, and increasing your PR isn't going to help you. I have some PR5-6 sites that get less than 2-3 visitors a day, but it is highly specialized traffic, and that's what I expect. Make sense? M
I agree with the other posters here. It really depends on the relevancy of your blog to the keywords being searched. Which keywords would you want your visitors to use and then use those keywords in your content. Also check in some keyword tool to see what are the keywords being used relevant to the topic of your blog and then use some of those keywords. I am sure you will start getting visitors for those terms. hope this helps.
Thanks guys . I think my main problem is that 3 of the 4 are photo blogs that have very little text and therefore keywords. But I still think there should at least be some SE traffic. I'm talking 2-3 hits a day. There has to be some good backlinks if they have a PR4. Would think that would count for something.