QB Issue Resolution:

it is one company per financial file – that is an IRS requirement
and each company must file it’s own tax return, even sole proprietors

and it makes sense, you do not want an auditor for company one, nosing around company 2, 3, and 4

personal finance is never mixed with the company file

Resolution for Issue 'How do I have multiple businesses and personal finances in quickbooks?' available: Yes (Solved).
Source: Intuit Community forum.

How do I have multiple businesses and personal finances in quickbooks?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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