Here is the scale:
KudoZ grading scale
When deciding how many points to award, please consider this scale as a guideline:
4: Answer was acceptable, explanation was good, reference was provided (or not needed)
3: Answer was acceptable, explanation was good, but reference was lacking
2: Answer was acceptable
1: Answer was somewhat helpful
You can see it clicking the small 'i' near the word "Points:" in the window opened when you grade an answer.