A Brahmin is always remains a Brahmin.
Vanaparva:CLXLIX:-Whether the Brahmana be cognisant of the Vedas or ignorant of them, whether they be pure or impure, they should never be insulted, for Brahmanas are like fires.
Parasara Smriti 8.11 “Brahmanas, who are ignorant of the Vedas, and Gayatri and Sandhya, and those who do not cast any oblation in ...
Chandogya Upanishad VIII.15.1 says (Swami Gambhirananda translator):
Brahma imparted this knowledge that was such to Prajpati, Prajpati (imparted it) to Manu, (and) Manu to the creatures. Having studied the Vedas in the teacher's house in accordance with the injunctions, during leisure after performing the duties to the teacher, he returns home from the ...
No there's no strict limit.
No, not at 4 are required to be learnt. It's as per your wish.
Yes, it is required but it is already done if one has done Upanayan sanskar I think.
This is not true. Each Veda has multiple Shakhas. Each shakha is complete Veda by itself. Although, it is true that many Shakhas have gone extinct.
Shudras are allowed to chant Vedic mantras.
यत्पाञ्चजन्यया विशेन्द्रे घोषा असृक्षत । अस्तृणाद्बर्हणा विपोऽर्यो मानस्य स क्षयः॥७॥
When the Five community (Brahmins, Kshatriyas, Vaishyas, Shudras, Nishad,) chant the mantras to praise Indra for rain when it is not raining, then happy to hear those eulogies For Indra rain, the cloud of the whole world is ...