I just realized that I shot a roll of Portra 160, while my camera was set on iso 400.
Is there something I should ask for at the developing lab to make up for it?
I hope that some of you might be able to help me out - thanks!
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Your camera "thought" it had film with an ISO of 400, while in reality it was only 160. So it adjusted it's exposure meter for ISO 400 film, underexposing your film by a bit more than 1 stop.
To compensate for this underexposure you need to ask the lab to "overdevelop" your film by 1 1/3 to 1 1/2 stops. This is called push processing.
I have no experience with this process, I just gathered the information from existing answers.