First off, pretty much all experts will say that "4 is the floor". Anything below 4 mmol (72) is considered low. To keep your blood sugar in the "normal" range you want to be less than 100 fasting (100 - 125 is considered pre diabetic and another over 126 is diabetic) and you want to keep your 2 hour post meal (post prandial) reading to be less than 140 (7.8). Some folks say 180 (10 mmol) but I think that is because a lot of docs have become lax and sort of infantilise their patients. They figure their clients just won't be able to do it and so set looser guidelines. I personally think that the closer to "normal" or non-diabetic you can get, the better off you will be.
That said, try not to be too obsessive (I know, easy for me to say). You will slip up once in a while and sometimes sugars will go up for no apparent reason. But if you see trends, and tend to start running higher and higher, then something needs to be addressed. Either diet changes, more exercise, or perhaps meds.
But in the meantime, don't forget to enjoy your life. I think it's great that you're putting such effort into learning about this disease and how you can best handle it.
Keep up the good work. Hope this answers your questions.
Cora