Hello W2, I haven't seen you for a while.
To my mind, the examples have the following nuances:
#1 states the simple fact of the length of time.
With #2, there's a suggestion that you might be about to have a haircut, or are thinking about it.
With #3, you are putting a little more emphasis on the length of time. That's what you want to express. (You might say it in a wondering tone, for instance.)
And #4 is a combination of #2 and #3.
But I may be imagining it. I'd be interested to hear what other people have to say.
MrP