Is the Zakat paid on 4,000 valid if the salary increases monthly and the balance exceeds this amount after one year?

Zakat (definition) becomes obligatory on four thousand if it reaches the nisab (definition) and a hawl (full lunar year) passes over it. The nisab is equivalent to 85 grams of gold or 595 grams of silver. As for additional salaries, they can be combined with the original amount and zakat paid on all of it at the end of the hawl (which is easier and more beneficial), or a separate hawl can be designated for each saved amount (which is difficult).