What does "the first, then the second, the third and the fourth" mean in Ziyarat Ashura?

It is stated in Ziyarat Ashura : > O my Allah, let the curse I call down on the head of the first tyrant stick like a leech; and stay put for ever on *the first, then the second, the third and the fourth*. What does "the first, then the second, the third and the fourth" mean?