I$400 will get you a nice Giant Escape 2 or a Trek FX 7.1 ..............
I'm familiar with those 2 brands and I also own a Specialized Sirrus which I really like, but its above $400.
Just go to your local LBS and ride the Giant or Trek or both. Go with the LBS that gives you the best support.
Good advice from TBO regarding lights, bell, air pump and a lock.