I have 4 different business units and 4 sales processes, how can I combine in the CRM?

If you capture the same kind of information for leads, contacts etc then you can use the same CRM account.

If you use the Enterprise edition then you can create different roles and profiles for the 4 business unit users and based on the profiles and roles the access permission can be set to vary accordingly between unit teams.

In addition, the field level security can be used to specify the access permission for different users.