```python
"""Render a value as fixed-width unsigned binary, nibble-grouped."""


def format_bits(value: int, width: int) -> str:
    if isinstance(value, bool) or not isinstance(value, int) or value < 0:
        raise ValueError("value must be a non-negative integer")
    if not (width > 0 and width % 4 == 0):
        raise ValueError("width must be a positive multiple of four")
    if value.bit_length() > width:
        raise ValueError(f"value {value} does not fit in {width} bits")

    binary_str = bin(value)[2:].zfill(width)
    nibble_groups = [binary_str[i:i+4] for i in range(0, len(binary_str), 4)]
    return ' '.join(nibble_groups)
```