To start of first: You want a hosting plan that can keep up with 400+ visitors at the same time, or spread over the day?

And don't take me on this, I'm not an expert, but i've seen this question a few times and I remember saying them: You need at least RAM on the machine, equal to the maximum -at the same time visiting your site- of visitors times (*) your webpage size.

Starting with 400 visitors (at the same time?) then i'd recommend the 20/mo plan. If you mean 400 visitors spread over the day, try the 10/mo plan and regularly check the site droplets graphs as the site visitors amount grows if it can still live up to it. ;)