Landsat data is in 8-bit (0-255) format. Would the difference between the sets of visible-NIR bands for Landsat 4,5, and 7 vary significantly?
Yes with some observations. A 0-255 range of DN is a ~0.004 step for each level of reflectance.
For NIR is enough, you can be able to identify a significant range of values into different land covers between 0 to 60% of reflectance. But for visible spectral range, water, vegetation and soil will be in a range from 0 to 15% of reflectance. This means that you can find ~40 different values in a 8-bit image.