If there are two of you, then rent-a-car is your best option - especially as you only have 4 days.
Don't rent when you get there though, book it in your own country. It'll be a lot cheaper for sure. Whether you're sailing into port or getting a flight in, they'll arrange for your car to be closeby for you.
Buses and trains in Norway are good and reasonably priced, but every trip will be two tickets and in 4 days you're limited to what you can see and do.
In a car (the roads in Norway are superb), once out of the big towns (Oslo and Bergen, to mention just two that I have been to, both absolutely beautiful), just head along the coast, or slightly inland. The mountains, waterfalls, lakes and fjords will have you stopping to take pictures every 5 minutes. Oh - and the sunsets???? Tearjerking.
One thing to pen in, traffic violation penalties in Norway are amongst the harshest anywhere on the planet. Then again, if you don't violate - you aint got nowt to worry about.
Lucky you 2.