build a massive quantity of free hosted blogs using thumblogger and the other big free blog hosts. I'm talking 5+ a day, with at least 10 quality hand written blog posts on them that target the keywords your 4 main blogs are using. Then once you've been doing this for a week, you should have at least 25 blogs that have quality content. Get them noticed with blog directory listings, and submitting individual posts to social bookmarking sites that are do-follow. You can also use linkdumps however SE's tend to think linkdumps suck.
I have a metric fuckton of traffic going to my free blogs alone, all done using the techniques above. I also have some paid-hosting blogs that are optimized for specific keywords, the same keywords that the free blogs are optimized for. All of my free blogs are interlinked in a sort of link wheel kind of setup, and they all point, either directly or indirectly, to my money blogs. The free blogs make money too, but we're talking hundreds of blogs that make the same amount as 5 or 6 "money blogs". The free blogs are nice because you build them up into something that's not a spamfest, and the you can just leave them, kind of like a freesite. You can use a content spinner that will rotate/re-write the posts for you and post a new one once or twice a month. Or you can do it yourself. It's all about a good schedule if you do the updates yourself. Make it so you're doing five or six blog posts a day to five or six free blogs, then do it again tommorow to a different group of blogs.
I see a lot of people trying to go far with blogs, but they get bored out of their minds and then quit in a week, after spending hours and hours a day making and/or updating blogs with real content that's actually worth reading. You won't make any money if you give up, so you just have to buckle down and be prepared to be bored as fuck for weeks and weeks until you have a good network going. The month that I did the majority of my free blogs is a month I cringe at when I look back. 10+ hours a day, never leaving the house, skipping meals, and wanting to toss myself out the window were the norm
But now all those suicidal thoughts and lack of showers are paying off, so it WAS worth it as of now... Looking back to when I was doing it, it was most definitely NOT worth it
PS: Now that you can compete with my blog kingdom, I demand 5 dollars for this advice.