I think it's generally accepted that a non diabetic would normally not expect to have their BS go below 4 and therefore a reading below 4 is deemed by most people to be a hypo.
That said, I personally don't think there are hard and fast rules.
If you manage your BS so that it is in the normal non diabetic range of say 4 - 8 then you will often feel an effect (hypo) when it goes lower than 4. But, if your BS range is higher you feel an effect at say 5 or 6 and sometimes even higher than that.
I personally find the opposite is also true, in that my BS is often around 3 - 4 for long periods and I feel little if any effect until it drops below 3. The difficult part is that some people will argue that a hypo is anything below 4 and a hypo is bad no matter how you feel. You will also get the argument that you simply are not noticing the hypo and it happens at anything below 4 no matter how you feel.
My thoughts on this 'often controversial subject' is that a hypo should be defined as the point you are not able to function normally. I don't think people should get so hung about the numbers...
You should aim to manage your BS as near to normal as possible, while still maintaining a good quality of life. i.e. don't beat yourself up if your numbers are a little higher than you would like.