To create a test case generator for the given problem statement, we need to generate a random string of exactly 12 uppercase English letters. Here is a Python function that generates such test cases:

```python
import random
import string

def tcgen():
    # Generate a random string s of exactly 12 uppercase English letters
    s = ''.join(random.choices(string.ascii_uppercase, k=12))
    
    return s

# Example use case
if __name__ == "__main__":
    for _ in range(5):  # Generate 5 test cases
        s = tcgen()
        print(f"Input: {s}")
        # Generating the expected output for our understanding (not needed in actual test case generation)
        expected_output = s[:4] + ' ' + s[4:]
        print(f"Expected Output: {expected_output}\n")
```

This function generates a random string `s` of exactly 12 uppercase English letters, which conforms to the constraints given in the problem statement.

Here's what the `tcgen` function does:
1. Uses `random.choices()` to select 12 random characters from `string.ascii_uppercase`, which contains all uppercase English letters.
2. Joins these characters into a single string `s`.

In the example use case provided, it generates 5 test cases and prints them along with their expected outputs. The expected output is calculated by inserting a space between the first 4 and the last 8 characters of the string `s`.