From Marseille to Denmark. Suggestions?

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