R1 = R2 = R3 = 6Ω

To find \( I_4 \), we need to analyze the circuit and use the given resistor values:

- \( R_1 = R_2 = R_3 = 6 \, \Omega \)
- \( R_4 = 3 \, \Omega \)
- \( R_5 = R_8 = 8 \, \Omega \)
- \( R_6 = R_7 = 6 \, \Omega \)
- \( R_9 = 10 \, \Omega \)
- \( R_{10} = 15 \, \Omega \)
- Total current \( I = 12 \, \text{A} \)

Steps:

1. **Calculate Equivalent Resistances:**
- In series, resistances are added: \( R_{\text{series}} = R_1 + R_2 + R_3 + R_9 \).
- In parallel, use: \( \frac{1}{R_{\text{parallel}}} = \frac{1}{R_6} + \frac{1}{R_7} \).
- Consider the parallel combinations involving \( R_6, R_7 \) and analyze how these fit into the circuit.
- Analyze the combination of \( R_5, R_8, R_9 \), and \( R_{10} \).

2. **Use Kirchhoff's Laws:**
- Apply Kirchhoff's Current Law (KCL) at different junctions to find individual currents.
- Use Kirchhoff's Voltage Law (KVL) in loops to establish equations for potential differences.

3. **Ohm’s Law:**
- Relate voltage, current, and resistance: \( V = IR \).

4. **Solve for \( I_4 \):**
- Use the information above to establish equations.
- Solve simultaneously to find \( I_4 \).

Without explicit calculation, the solution process involves these principles and setup equations according to the circuit analysis. The methodology will lead to finding the value for \( I_4 \).