Wow, thanks. This solution was different than I expected 
Never worked with loops etc.
As I never want to just copy solutions, but really understand them, I tried to also find an own way.
Inspired by your loop, I thought what if I just split the task in 4 different sequences, rename the columns and then append them again. Some kind of a āmanualā loop.
This is what I did:
Seems it worked as well. Result:
I am really happy with that.
Question here is: How do I get rid of all the other columns? Or is that not important if they all appear there, because in the POST Request node I can for example select a specific column for my body, and thus I would just select this last column called āFinalJSONāā¦?
This is the full picture from the last node (Node 8, Columns to JSON):
Btw: When I mark an answer as solution, will this thread then automatically be closed?