Depends on what you want.(posted 8522 days ago)Here's the short version: If you want shadow detail and not too much contrast, use true ISO 400 film. If you don't care about shadow detail, and can live with the increased contrast pushing entails, push.
Pushing does not increase the true speed of the film (at least not notably, maybe by 1/3 f-stop or so), but it does increase contrast considerably. Therefore, there will not be any detail in the shadows if you use pushed film, and the negs will be rather contrasty. It's not the thing to try when your subject is already contrasty.
True ISO 400 film, on the other hand, will still have detail in areas that read about two f-stops below the midtones.
Regards, Thomas Wollstein