It is obviously pretty subjective, but I can tell you from experience that the only way you will know is to practice. Once you have seen 4+, you'll know it, as the leg practically flys off!!!
Here's how I define them:
1+ : foot moves
2+ : small bounce, the "general" reflex you will usually get during routine assessment.
3+ : what I call "hyperreflexic", where it is a definite kick
4+ : don't get too close to the leg or you will be kicked in the face
I also notice that when they are 3+ to 4+ is when I can just tap with my fingers and get a huge response in reflex form.
About clonus, I have been in OB (small hospitals, low risk usually) I had NEVER seen TRUE clonus until about a month ago. Two beats, was awesome! (not for the patient, who made me very nervous with BP's of 196/110 and 4+ protein in urine 2 days PP)
I have been told by more experienced OB nurses that there IS "non-true" clonus, where you will get a little bounce afterwords, usually from someone who is very anxious, and has only 2-3+ DTR's....
In time, you will figure it out. What I did when I was learning was to have more experienced nurses, or even the docs, check after me and compare results. And even if I had TWO different people check after me, the results varied. I could have called it 3+, and had another nurse call it 4+ and another one call it 2+...
To expound, the patient that I felt clonus on and 4+ reflexes, the doc came in right after and said she had only 2+ and no clonus... and the nurse working with me agreed with my assessment as well.... (me thinks the doc just didn't want to start mag on her, personally)