I actually solved this by creating a formula by level by field (so in my case, 4 formulas for each field).
In the formula, I check if the subsequent level is null or not to actually know if the value should be displayed on this level or the next one:
Comment_Level3 = if(isnull(Level4), if(isnull(Comment), '', Comment), '')
I don't know if that's the optimal solution, but it could be helpful to someone who's facing the same.
Or if someone could share a better solution, this would be great.