The main four Marja's in Najaf Ashraf are:
1. Aytullah Seestani who is followed by the majority of the Shia Muslims all over the world.
2. Ayatullah Sayyed Mohammad Saeed al-Hakeem.
3. Ayatullah Shaikh Eshaq al-Fayadh.
4. Ayatullah Shaikh Basheer al-Najafi.
The Marja' who is been followed by the majority of the Shia is called ( Al-Marja al-A'la) ( The highest Marja).
We have a system of our religious leadership based on highest degree of knowledge and piousness, which is quite different from Vatican.