Can I upgrade to more than 2tb?

There isn't a 4TB plan as such; you have to do it by having a subscription to Apple One and also subscribing to an individual iCloud+ plan.

Refer to this document about how to do this --> What happens to your iCloud storage when you sign up for Apple One - Apple Support

"If your family needs additional iCloud storage, the family member who purchased Apple One can buy more iCloud storage. With both Apple One and an iCloud+ plan, your family can share up to 4TB of total iCloud storage."

Apple just announced new storage options at Apple’s iPhone event on Sep 12,2023. The storage options are 6TB for $29.99 and 12TB for $59.99