I presume from your question that the 4 business units span the 3 companies.

You could use analytic accounting to hold the business unit information at a transactional level.

Then have a group company that rolls up the data from the three sub companies and provides consolidated reporting. You would need to test if financial reporting by analytic account works at the parent company level.

First of all, set the companies with correct structure. You should choose a parent company for each if you want to give them a hierarchical structure. There are no business units in Odoo. Permissions to access multiple companies are user based so you need to configure which people are going to see and edit which companies in the user's permission page. And give them appropriate permissions. You may need to create special groups (roles) to acquire desired permission list. I haven't tested if analytic accounts work as Kevin suggested but you may need to add a top holding company, even if it doesn't exist, if you need to consolidate the data.