Final answer:
Internal auditors of a company can provide assistance to external auditors and reduce costs, and their work can be relied upon even if they're not CPAs. They generally do not express an opinion on financial statements, which is the role of external auditors.
Step-by-step explanation:
If a company has an effective internal audit department, internal auditors can perform several key functions. Notably, they can provide direct assistance to the external auditors, which may help reduce external audit costs. Additionally, their work can indeed be relied upon by the external auditors, even if the internal auditors are not CPAs, provided the external auditors perform adequate oversight and consider the internal auditors' competence and objectivity. Contrary to the idea presented in option 4, their work can be used by the external auditors, as per the standards set by organizations like the PCAOB, which allows external auditors to use the work of others in certain circumstances.
However, internal auditors typically do not express an opinion on the fairness of the financial statements; this responsibility lies with the external auditors. The internal audit's role is to provide independent assurance that the company's risk management, governance, and internal control processes are operating effectively.