400 doesn't need much of preparation. All you need to do is to concentrate on two topics(in both sections verbal and quant) that are tested heavily in the GMAT.
For Quant: practice a lot on Number Properties and on RTD(rate and distance).
For Verbal: modifiers for sentence correction and critical reasoning ( I excluded reading comprehension because it needs a lot of practice).
Overall: you have to study a little bit otherwise your score will not change.
this forum is incredible resource. Search in the forum for specific questions such as modifiers or number properties(using Tags list by forum link, which you can find at the top of the page).