Sure, here's a test case generator for the problem described:

```python
import random

def tcgen():
    # Generate a string of length 4 consisting of '+' or '-'
    S = ''.join(random.choice(['+', '-']) for _ in range(4))
    return S

# Example usage:
for _ in range(5):
    print(tcgen())
```

This test case generator produces a random string of four '+' or '-' characters, which can be used to simulate different inputs for the problem. Below are a few sample outputs from running the generator:

```
-+-+
+--+
++--
++++
--++
```

You can use these generated test cases to test the implementation of the solution for the given problem.