A holding company is simply a vehicle for the ownership of a range of other companies.
Company 1 and 2 are wholly owned subsidiaries or the holding company owns a majority of shares.
Companies 3 and 4 are part of the group but the company does not own the majority of shares.
The holding company manages a portfolio of companies and does not involve itself in the management or strategy at strategic Business Unit level (SBU). Thus it provides few synergies, and little in the way of corporate services or parenting roles.
This has advantages in that the holding company does not become enmeshed in complex logistical arrangements that would make divestment difficult if this was in the shareholders best interest.
The advantage for SBU is that they can continue to manage autonomously, and may have opportunities to raise capital more cheaply through the group than by using the money markets.
The problems are that the SBU might be divested for reasons entirely unconnected with its own performance, and there are a few opportunities to leverage core competencies across the group structure.