The fan kicked on for the first time this year just yesterday. I think its time to bring this to the next step. I just ordered
a 6" inline duct fan. This fan blows 440 cfm. The three 4" fans blow 120-150. So, this should be pretty equivalent to the three (only 2 working) fans that I have been using. It is also is an inline fan, so I will be hanging it and this will isolate a lot of vibrations and hopefully quiet things down which will be a great side effect.
This is the fan I got: