This first poem of "Four Quartets" says a lot of stuff about how the past, present, and future all exist at a single moment, which is whatever moment we're living in right now. The idea that the past and future are somehow different from the present is a symptom of the fact that we have trouble focusing on the world around us in the present moment. If we're going to have any hope of getting back to the stuff of life, Eliot suggests, we're going to have to start thinking in the "here, now, always" (180).
This poem expands on the themes of "Burnt Norton" and shows us that our lives are not ruled by clocks and calendars, but by the changing of the seasons and nature in general. Cultures of the past, Eliot suggests, were more connected to nature, and this is something we should envy. At the end of the day, there's no point in obsessing over our human-made accomplishments. We're all going to die someday, and the best we can do with the time we have is look for "a further union, a deeper communion" (389) with nature and with the present moment.
Eliot really starts swinging for the fences in this one, especially when it comes to educating his readers on the value of letting go of their individual egos. All of our modern pain, Eliot suggests, is in some way connected to the way we think of ourselves as goal-driven individuals instead of as part of a larger whole. In this section, Eliot also levels with us and admits that letting go of our egos is probably going to be a really painful process. But he ends the section on a note of encouragement, saying that "We are only undefeated / Because we have gone on trying" (634-635). Rather than complaining about the modern world (as he does in "The Waste Land"), Eliot actually comes pretty close to giving a pump-up speech (though maybe not as intensely as this guy).
In this final installment of "Four Quartets," Eliot focuses mostly on how drastically our thinking will have to change if we're going to have any hope of achieving the spiritual rebirth we're supposed to be looking for. Ultimately, he demonstrates that, in order to change the world, we'll first have to change ourselves, and one of the hardest things we'll have to do is move beyond thinking of the world in oppositions like up-down, past-future, self-other, etc. We need to find a new language for talking about experience, because the old language isn't working anymore. Eliot is surprisingly hopeful about our ability to change, though he does keeping admitting that real change can only come with a bit of pain.