1

You are wasting your money going to Oslo thinking you might see Northern Lights, its too far from the right latitude.

Northern Lights are observable in the vicinity of Oslo on just a handful of occasions per year, maybe a bit more in an active period like recent times. There's no point making a trip to Oslo thinking you all you have to do is get to some dark place with clear skies and wait a couple of days, you are 2 hours flight south of where such a strategy is realistic. If you are in Oslo, and then you hear in the news that there is some big display on that can be seen as far south as Oslo, and there is a prospect of clear skies, then by all means look into getting out of town to somewhere dark.

The best place for northern light spotting in Norway, in near vicinity to an airport one stop from Oslo, is Alta. But best check the weather forecast if you are only going for a short visit, because there is no point going if its going to be cloud all the time.

Stockholm and Helsinki are almost equally useless btw. But you might find northern Sweden or northern Finland cheaper to visit.

2

With only four days to play with, your chances are less than 50/50 wherever you go. And it won't be cheap.