Going from 400 to 40,000 visitors/day is a hard task if you want this traffic to be organic. You should start promoting, doing some SEO, experiment with your site since you are new to this (otherwise you wouldn't ask about this). If you see your traffic increasing let's say from 400 to 500 or 600 then you are on the right way. It is going to be a long procedure until you will be having 40,000 visits/day. Aiming high is good but try to be logical and set achievable goals. Work on getting your traffic to 1,000 organic visitors/day first. When you achieve this you will know what to do to go further. As I said, promote, buy advertising space on other related sites, do some SEO and give your visitors a reason to keep coming back. There is no secret in how to do this. Either you do this through trial and error or you pay someone to do this for you.
need traffic = buy traffic. (joking) if you need a quality visitor you need to make an analystic. who is your target visitors, what do they want from your site, where are they from, where to find them, who is your competitor, and etc. then you will come up with the concrete plan how to get more traffic.
Some niche's have cieling though of how much traffic it can get. It all depend on number of search type ins. So organic traffic from the search engine is what you'll want. With that you have to find the keywords you think will pull in traffic and build tons of backlinks with that anchor text to pull your site up in the serps to acquire more traffic.