What is the amount due for Zakat for multiple years: Is the nisab calculated for each year separately, or is the annual income combined and multiplied by the Zakat percentage?

If you have not paid zakat for four years, you must pay zakat on the money for all the past years, by calculating the money you had at the end of each year. This method is easier, but if the money was earned from a salary, then some parts of it may not have completed a full year (hawl), which is a condition for zakat to be obligatory. The other method, which involves calculating a separate hawl for each month, is difficult.