The linkblog can be included in the sidebar of the other blogs if you like. Go to the blogs tab, select one of your blogs, then select the linkblog in the dropdown box labeled Default linkblog. Save your settings, then anything you post in the linkblog will show up in the sidebar of the other blog. You could set your first three blogs to use the fourth blog as the linkblog. Or you could create a new linkblog for each of the first three blogs.

If you don't see it in the sidebar after changing your settings, then the skin you're using might need to be edited to include the linkblog code. You can just copy the relevant code from the custom skin.