Ziyarat Ashura is an authentic and reputable hadith narrated from Imam Muhammad Baqir a.s. the 5th infallible Imam and one of the Ahl al Bayt who loving and obeying them is obligatory for Muslims. This hadith is narrated from 5th Imam by different reputable chain of narrators.
It is not mentioned in a any reputable hadith that who are the first, then the second, the third and the fourth in Ziyarat Ashura. In fact it is as it is. It is not defined and no one can say certainly who they mean in this hadith.
Shaykh Tusi in his book Al-Istibsar (the fourth important book of Shi'a Islamic Hadith) has said that the first means the son of Adam who murdered his pious brother. The second is Qeydar who hamstrung and murdered the Camel of God and the 3th is who murdered prophet Yahya Ibn Zakariya a.s. (John the Baptist who is mentioned in different verses of Quran like 6:85 and 19:12) and the 4th is Ibn Muljam who murdered Imam Ali a.s. while Imam was at prayer (Ibn Muljam is called the most wretched human in different Ahadeeth). But this opinion of Shaykh Tusi and not narrated as a hadith.
http://www.pasokhgoo.ir/node/51811 (the official website of Islamic Questions answering center of hawzeh Qum)