Stanza 4 Summary

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

Unwearied still, lover by lover,
They paddle in the cold
Companionable streams or climb the air;

The fourth stanza opens with another description of the swans. They are "unwearied still" (as in, not tired or worn-out yet) and swim in the water and "climb the air." Man, these swans have some energy! (Frankly, we need a nap just reading about them.)
"Lover by lover"? What's that about? It seems like the swans all have companions. They swim together in the water, perhaps in pairs as if they were lovers.
We're guessing that this companionship makes the cold water more bearable.
This connection is carried forward with the notion of "Companionable streams." Even if the water is cold, it's welcoming to these love-birds. It's as though the speaker imagines that the swans themselves were meant for each other as they paddle through the stream, or fly about.
So, we've got swans that are energetic, and that are paired up. Must be nice! But we wonder if the speaker is making a contrast with his own condition…

Their hearts have not grown old;
Passion or conquest, wander where they will,
Attend upon them still.

When the speaker talks about hearts growing old, he's using another synecdoche, describing a part to mean the whole. So, it's like the swans themselves have not grown old. Good for them! Nope, wherever these swans go, "Passion or conquest […] / Attend upon them still." Great, great. Um, come again?
What the speaker means here is that these youthful swans seem to feel passion, or the energy for conquest. That kind of enthusiasm "Attend[s] upon them."
In other words, they're still being visited by this sort of vitality. (Personification alert! To say that something abstract like passion or conquest might attend on you, like a waiter at a fancy restaurant, is to lend it human qualities.)
We can't help but consider how their youth and exuberance is starting to contrast with our speaker's age (or sense of age), though.