SharePoint Online Storage Limits

Ravindra,

     As others have said, that would be supported based on the SPO limits defined by Microsoft.  I actually have a few site collections in one of the tenants I manage that is approaching 300,000 items in a single library (but not anywhere near 4TB...yet). While this is definitely supported, if you plan to put that many items in a single library, you do still need to be aware of recommendations around management of large lists in SharePoint - https://support.office.com/en-us/article/manage-large-lists-and-libraries-in-sharepoint-b8588dae-9387-48c2-9248-c24122f07c59.  While the Modern UI in SharePoint does make it better and more management, you do still need to do some thinking/planning around those large lists, especially if it's interfacing with a third party product or you're accessing the list in a way other than directly through the SharePoint UI.

Another aspect you mentioned that I would think about is the use of Subsites. I would recommend not doing subsites, but rather doing multiple site collections and then leveraging hub sites. Microsoft is moving away from the whole concept of subsites as you can see by some of the updates to SharePoint Online settings (to disable the creation of sub-sites) and features they are coming out with (again hub sites).  If you are going to break this up, break it up across multiple site collections, not a single singe site collection with multiple subsites.

Just my two cents...