geeez dude, that's IMPOSSIBLE to answer! YOU will have to build the site and try it all out up and down. NO ONE can answer you that. So get to work and signup with the sponsors and try. You have 4M/mo hits and you are asking if the site is even worth building? SURE it is. But it TOTALLY depends how you will filter that traffic and the send it to the sponsors (what ads you will be using and how). It's all trial and error.
And BTW, word "free" is a nice word. Filter folks that are searching for "free" and send them to a sponsor with free trials (check my signature) for example.
-- McSpike --
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CONVERSION CASH --> Converts like a motherfucker! FREE trials, $35 per converted join + 50% on every rebill!
People are an easy sell when it comes to FREE trials 