Stanza 4 Summary

Lines 19-21

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…

Lines 22-24

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.