Kodak Portra 400 is a pro film by Kodak, meaning it has lots of latitude for underexposure and overexposure. Generally speaking when it comes to color film you want to overexpose.

However, you can get good results at box speed! That means shooting it at 400 and developing at 400. One of the results of overexposing color film is you get deeper saturation in the colors and unlike digital photography, you don't have to worry so much about blowing out your highlights.

Now there are a couple of ways you can go about doing this. The easiest way would be set your ISO dial to 200, 100, or maybe even 50, depending on what your camera is capable of. It’s the easiest way because your light meter and ISO is compensating for the exposure.

The second way of achieving this is by overexposing like you would if you were shooting normally overexposing it manually while looking through the camera.

Quick recap: overexpose the film by one, two, sometimes even three stops to saturate those colors. Don't worry about the highlights. Send it in to your film lab and have them develop it at normal box speed (400).

And now you got the best results you can get with Kodak Portra 400.