Patrioti I have what you describe, I tried to upgrade my plan and there is no option, at least here in Sweden to upgrade my Apple One to a higher level, it has only downgrade to personal, but I have a family plan and wish to keep it that way. Currently we are sharing 2.2TB there is nowhere to be found on my devices on how to upgrade to a higher capacity.
Never said there is a 4TB plan, that is why I wrote "4TB (2TB Apple One + 2TB iCloud+)"
I have the 2TB iCloud+ shared with the family and the Apple One plans I see on my phone is Individual Apple One with 50GB included and Family Apple One with 200GB included, which is my current plan and the only option is to *downgrade*
I hope this makes it more clear.