I would like to put these “tracks” in a series but I don’t know which entity (recording? release? release group?) to add to which kind of series (Recording-Series? Release-Series? Release Group-Series?)
I think you have a Release Group Series here. You are gathering together multiple release groups to show they are in common.
This artist may decicde to release some of these as CDs. Which is why you’d want to keep this a Release Group series with four members. It is the fact they are being released is interesting.
Yes, they do also fit a Recording Series. That would be useful if this appear later on a compilation. That would make sure you only have the four members of the set.
Whichever type of series you create please also add a good waffly annotation to explain it. Especially something as special as this.
Now for an example of release series where it’s pretty self explanatory why it cannot be a release group series as these albums did already exist in their own: Bob Marley & The Wailers: The Definitive Remasters
This should be release group series, but I assume that because of all the existing series-release relationships, I cannot modify the type anymore, right?