One possible way is to look out for advertisers on your site. Since you have a PR4 site, there should be quite a lot of websites that would want to place a link on your site to make use of the Google juice. How about placing a link 'Advertise here' and see if someone is willing to advertise...
Well do what everyone else seems to do say OOH it's PR4 everyone should buy a link on it! This is the method used opposed to actually planing to make long term money from the site.
best idea ... try selling text links. and don't stop your website's developement. try to improve it, maybe at next PR update you'll get PR 5, and so on.