This circuit implements the Deutsch-Jozsa algorithm for a balanced function and measures the first three qubits of a four-qubit system.
```python
from qiskit import QuantumCircuit
# Create circuit with 4 qubits and 3 classical bits
qc = QuantumCircuit(4, 3)
# Initialize the last qubit to |1>
qc.x(3)
# Apply Hadamard gates to all qubits
qc.h([0, 1, 2, 3])
# Oracle for a balanced function (example for Deutsch-Jozsa algorithm)
qc.cx(0, 3)
qc.cx(1, 3)
qc.cx(2, 3)
# Apply Hadamard gates to the first three qubits
qc.h([0, 1, 2])
# Measure the first three qubits
qc.measure([0, 1, 2], [0, 1, 2])
```
{
"000": 8,
"001": 4,
"010": 4,
"011": 16,
"101": 20,
"110": 45,
"111": 927
}