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


def format_bits(value, width):
    if 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 != 0 or width > 32:
        raise ValueError("width must be a positive multiple of four and not exceed 32")
    if value > (1 << width) - 1:
        raise ValueError("value does not fit in width bits")
    return format(value, f'0{width}b')[::-1].group(4)[::-1]
```