Stanza 4 Summary

Get out the microscope, because we’re going through this poem line-by-line.

I shall be telling this with a sigh
Somewhere ages and ages hence:

Now we jump forward in time. We don't know exactly when, but we know that it's ages and ages "hence," or, from now. So we're probably talking years, not months.
We know that this story is important, because the speaker will still be telling it many years later.
He'll be telling it with a sigh, though, which is interesting because sighs can be happy, sad, or merely reflective – and we don't know what kind of sigh this is.
So, we know that this choice is probably going to be important for the speaker's future, but we don't know if he's going to be happy about it or not.

Two roads diverged in a wood, and I—

This line is a repetition of the first line of the poem, with the subtraction of the word "yellow" and the addition of the words "and I."
This repetition helps to bring the poem to a conclusion. It reminds us what's important in the poem – the concept of choosing between two different paths.
Then, we get the hesitation of "and I" and the dash. This lets us know that whatever the speaker is about to say next is important.

I took the one less traveled by,

In this line, the speaker sums up his story and tells us that he took the road less traveled by. With the hesitation in the line before, this declaration could be triumphant – or regretful.
Also, remember it wasn't exactly clear that the road our speaker took was the one that was less traveled. He said at first that it looked less worn, but then that the two roads were actually about equal.
Before you start getting mad at our speaker for stretching the truth, remember that he's telling his story far in the future, a long time from when it actually happened. He's predicting that his memory will tell him that he took the road less traveled by, or that he'll lie in the future, no matter what the reality of the situation was.

And that has made all the difference.

At first glance it seems that this line is triumphant – the narrator took the path that no one else did, and that is what has made the difference in his life that made him successful.
But he doesn't say that it made him successful – an optimistic reader wants the line to read positively, but it could be read either way. A "difference" could mean success, or utter failure.
Remember, the speaker is telling us about what he's going to say in the future. From where he is now, just looking down the path as far as he can see, he can't tell if the future that it leads him to is going to be good or bad. He just knows that his choice is important – that it will make all the difference in his life.
The speaker of this poem could be saying that his choice made all the difference while he's surrounded by his grandchildren, by a fire in a cozy little house. Or he could be saying it to the wind, while walking alone on the streets. At this point, he doesn't know – and neither do we.