Explanation:

Box 1: No
Four containers with 1000 RU/s each.
Box 2: No
Max 8000 RU/s for db2. 8 containers, so 1000 RU/s for each container.
Box 3: Yes
Max 8000 RU/s for db2. 8 containers, so 1000 RU/s for each container. Can very well add an additional container.
Reference:
https://docs.microsoft.com/en-us/azure/cosmos-db/plan-manage-costs
https://azure.microsoft.com/en-us/pricing/details/cosmos-db/