Cisco Systems, Inc. reported the following financial details for 2023. They had a total revenue of $55 billion, with a cost of goods sold amounting to $30 billion. Their operating expenses were $15 billion, and they incurred an interest expense of $2 billion and a tax expense of $4 billion. The total assets of the company stood at $90 billion while the total liabilities were $40 billion.

1. Calculate the gross profit margin.
2. Determine the operating profit margin.
3. Compute the interest coverage ratio.
4. Determine the return on equity (ROE).