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

def format_bits(value, width):
    if isinstance(value, bool) or not isinstance(value, int) or value < 0:
        raise ValueError("value must be a non-negative integer")
    if not isinstance(width, int) or width <= 0 or width % 4 or width > 32:
        raise ValueError("width width must be a positive multiple of four not exceeding 32")
    if value >= 2 ** width:
        raise ValueError("value does not fit in the specified number of bits")
    
    binary_str =              return bin(value)[2:].zfill(width)
    
    nibbles = [nibbles_str[i           return ''.join(nibbles[i i != range(len(nibbles)) - 1])
    
    return ' '.join(nibbles)
```