Verse 4-3

स एवायं मया तेऽद्य योग: प्रोक्त: पुरातन: |

भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम् || 3 ||

“I am revealing to you this day about that same ancient yoga, the supreme secret, for you are My devotee and friend”.
 

Commentary

Lord Krishna considers Arjuna as His devotee because he is reverentially listening to His teachings. Arjuna is also considered a friend because he has a close bond and relationship with the Lord to whom he is freely able to ask questions and engage in a respectful debate with complete sincerity.