Gas mark 4 in a fan oven is equivalent to 160 degrees Celsius or 320 degrees Fahrenheit. Its important to note that fan oven temperatures are typically around 20°C less than conventional ovens because the fan circulates hot air around the inside of the oven and into the food, ensuring a more even temperature and food thats cooked more quickly. Therefore, when converting temperatures between fan and conventional ovens, its necessary to adjust the temperature accordingly.