Specialized and Trek both have pretty good options in that price range. You're not going to find anything top of the line for $400 (my race wheels alone cost twice that much) but you will find a good reliable bike. Look at the specialized hardrock and the Trek 4300 (I think that's the one near $400)
Or look around for a used bike. You may be able to find a 2 year old $1000 bike for near that price, if you shop around. Just make sure you aren't going to have to spend $200 as soon as you get it to replace all the worn parts. There are lots of people who buy nice bikes and never ride them then sell them cheap, barely used, so look for one of those.