The correct answer to the question is option 4, Wali. The term Wali was used by the Sufis to refer to the successor who was nominated by the teacher of a particular Sufi order or silsilah.

1. Option 1, Pir, refers to a spiritual guide or master who is highly respected and followed by the disciples. While a Pir plays an important role in guiding the disciples, they are not specifically referred to as successors nominated by the teacher.

2. Option 2, Murid, refers to a disciple or follower of a Sufi order. While Murids are an integral part of the Sufi tradition, they are not specifically referred to as successors nominated by the teacher.

3. Option 3, Khalifah, generally refers to a Muslim leader or ruler. In the context of the Sufis, it can refer to the disciple who is given authority by the teacher to guide others in their spiritual journey. However, it may not specifically indicate the successor nominated by the teacher.

Therefore, option 4, Wali, is the most accurate term used by the Sufis to describe the successor nominated by the teacher of a particular Sufi order or silsilah.