That's one of those "it depends" questions.
Non-diabetics can regularly go down to the high 3s before meals, and no one thinks anything of it, but then they are not at risk of going lower and into irrational/potentially comatose hypoland.
Personally, I get some symptoms in the low 4s so I don't lose sleep as long as there is a 4 in front of the decimal point, but...
I've had periods in the past when I've lost hypo awareness due to keeping my blood sugar too low, so I really really want to minimise the number of times I go below 4 and if I start to feel I'm losing awareness I wouldn't hesitate to aim for readings in the 5s and 6s for a while.
So it really depends on the individual, and their body's reaction to hypoglycemia.
If I could sit on 4 without going hypo I wouldn't have much of an issue, but the fact is that if I sit on 4 I'm likely to go below that, and life is too short to spend it having hypos all the time. (Well, my life is, and I loathe hypos with a passion.)