If you are starting from Stockholm, i would suggest going north in Sweden. Either E4 along the coast northbound (but the first 2-300 km is just a boring freeway) and follow the botnia coast as far north as you have time. Or go inland towards Dalarna and Siljan (Mora, Rättvik) and then north into the mountain areas of Harjedalen and Jamtland. Possibly head over to the Norweigan side via Åre/Storlien and then go south through Norway. Or combine all these, depending on how much time you have
If you are limited to the 300 km range and 4-5 days i would say the second alternative (Dalarna/Harjedalen) would be nice.