| Quote #4
It avails not, neither time or place – instance avails not,
The speaker sends himself into the future on a mission: to make friends with us and empathize with all our little problems. For him, time doesn't "avail," that is, it doesn't succeed in putting up obstacles between people.
| Quote #5
Others the same – others who look back on me, because I looked forward to them,
He explains more about how this connection to the future works. From the reader's perspective, our "looking back" on Whitman is what makes it seem like he is "looking forward" by speaking to us. But he's like, "No, no, I was the one who made the first move." Also, he makes it sounds like he exists in the present only by choice, because he chooses to "stop here to-day and to-night."
| Quote #6
What is it, then, between us?
The implied answer to these rhetorical questions is: nothing. What's a couple hundred years between old friends who've never met each other?