Hi,
Yes, you will be able to see all 400 extensions. But you cannot "manage" 400 extensions, you can only perform transfers and originations from your own (logged) extension. Example: you are logged as extension 100, you can originate calls from 100, and transfers calls from 100 to any other extension. But you cannot originate a call from 200, or transfer a call from 200 to 210.
Best regards,
