It can be tempting to increase a medicine's dose when the expected effect is not immediately noticeable. With modafinil, however, dose adjustments should be made as part of a medically supervised treatment plan.
Australian prescribing information allows doses to be increased in certain circumstances, up to a maximum of 400mg per day, but this does not mean that everyone should use the higher amount. It specifically notes that 400mg has not demonstrated statistically significant additional benefit over Modvigil 200mg for the relevant indications.
Taking more than prescribed can also increase exposure to the medicine and may increase the possibility of adverse effects.