@sreshtha Try to understand that k is a variable. k can take any positive number any time. But n is, like specified, constant for a particular language. So, when you take n=1, you need to make sure that the automata you make satisfies all values of k. I hope I clarified your doubt. Just think about it as a^(4k) OR a^(5k)....you need to make an automaton which accepts n number of a's, any number of times....matlab ki, a^n, a^2n, a^3n......for all k. Feel free to ask if you don't get it yet.