Portra 400 pushed 1 stop will add some contrast, some slight color shifts, and if you meter correctly the grain shouldn’t be that much more noticeable that when shot at box speed.
Example of Pushing Film 1 Stop
The pushing or pulling process involves shooting and developing film at a different ISO then what the film is rated for. Let’s say you want to push your 400 ISO film 1 stop (+1), set your camera to 800 ISO and shoot/meter the whole roll as 800 ISO. When the roll is finished, clearly mark the speed on the film canister. Marking the canisters is for your reference in completing the order form and letting the photo lab know how to adjust development time.
Read about Everything you need to know about Shooting and Developing Pushed or Pulled Film