可以使用以下代码将四元式元组列表转换为带换行的字符串:
```python
quads = [('=', '3', '_', 'T0'), ('*', '2', 'T0', 'T1'), ('+', 'R', 'r', 'T2'), ('*', 'T1', 'T2', 'A'), ('=', 'A', '_', 'B'), ('*', '2', 'T0', 'T3'), ('+', 'R', 'r', 'T4'), ('*', 'T3', 'T4', 'T5'), ('-', 'R', 'r', 'T6'), ('*', 'T5', 'T6', 'B')]
text = ''
for quad in quads:
text += ','.join([str(s) for s in quad]) + '\n'
print(text)
```
输出结果为:
```
=,3,_,T0
*,2,T0,T1
+,R,r,T2
*,T1,T2,A
=,A,_,B
*,2,T0,T3
+,R,r,T4
*,T3,T4,T5
-,R,r,T