The obvious routing would be Marseilles to Lille and on to Brussels. From there you need to head for Cologne and Hamburg. Then on to Copenhagen. I assume you are using the whole 4 days to get to Norway so any stops onthat routing would suit you e.g. Lille, Cologne or Bonn and Hamburg would give you reasonable train journeys and a wide choice of accommodation.

Hmmm, there are quite some possibilities. When do you plan to leave exactly?

There’s the one @Yorkie suggested,

Or possibly some options via Strasbourg/Karlsruhe/Frankfurt, directly from Marseille too, bypassing Belgium. From there it’s fairly easy to get to Hamburg, either via the Rhine-Ruhr-area, or via the faster line over Kassel and Hannover.

Getting to Copenhagen, From Brussels is doable, but a very long day (best includie some delay buffer too, preferrably in Hamburg, as trains to Denmark are not super frequent).

As you only have 4 travel days, it’s going to be quite long travel days whatever the stopover option you will choose.

Not necessarily long travel days but yes you'll need to travel a bit every day.

DAY 1

Marseille - Strasbourg direct TGV (10€ seat reservation fee) 08:11 departure (5h32)

DAY 2

Strasbourg - Offenburg - Cologne

For example 09:20 departure (3h44)

DAY 3

Cologne - Hamburg 4h

DAY 4

Hamburg - Copenhagen 4h45