You can do an exchange.

As to Hamburg - Narvik: it makes it easier if you just post your timetable including all trains. In general you need a new travel day if you change after midnight. Or differently put: only the date of departure matters.

That would likely be 2 tage-certainly the app counts very strict. IF you board at HBF-buy HVV till Altona or where it stops next and start from there to save tr.day.

IF you can reach the overnight from STO same day, it would count as just 1 day. reverse will never work that way.

Exchange as such is not possible-plus you did not state from who you bought. It means cancel the 4 day -pay fee for that, wait long till money refunded, and buy new 7 day. Refund always only via place that sold it to you. (could also be DB itself)

Not really. 23:59 is the departure time of the night train to Stockholm. Next stop: Høje Taastrup.

wow guys. thank you so much for your support!!

how can i cancel the 4day pass. send my pass number to an email…?

You can cancel your pass only via Costumer Service if you have purchased it online via Interrail.eu

Cancelling a pass bought via eurail.com is possible from Cancellations and refunds in your Eurail account.