Hi Jean,
First of all, your plan sounds good
You can create an organization with 4 members with your personal user account, the organization will have its own billing account, which means a separate invoice.
Your personal account and the accounts of the other 3 member can be either community or pro accounts, since any personal user can be a member of your organization.
Organizations have 1 GB of storage included. Any additional storage has to be prepaid and has a fixed price of 5€ per month per GB. If you need 2GB in total, you have 1GB from your plan and 1GB of additional storage. The additional storage will cost you 5€.
The storage is the sum of the size of all files in all projects that are owned by the account (In your case the account of the organization). Therefore, storage capacity of each member is not inherited from the organization’s total storage capacity.
Without creating an organization 4 members can not collaborate on the same project(s) only if the project is public. Public accounts can not have additional storage. Pro user can collaborate with one other pro user (not 3). The option -team management- is only available for organization accounts only.
Here are some further information about organization accounts:
To add a new organization member, you need to have their username or email first. Which means they need to register their own users.
When QFieldCloud charges the monthly bill for your organizational account, you pay only for the active organization members. By active, it’s someone who either downloaded or uploaded to a organizational project. If the organization member did nothing in during the month, it is not considered active.
Note there is a minimal amount of 2 users that will be billed each month, no matter if there were active.
See also our FAQs
Or other discussions like #3836 or #3832
If you upgrade till the 22nd of February you can get the EarlyBee
discount, which means 40% of the yearly price (instead 20€ - 12 € per user / month). The discount will automatically apply for the next 12 month.
I hope I could help! Enjoy QFieldCloud 
Best,
Zsanett
Imported from GitHub comment by @ZsanettMed on 2023-02-20T08:57:25Z