Stanza 4 Summary

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

Flashed all their sabres bare,

Keep in mind that these guys weren't carrying machine guns. They were riding through this storm of bullets, on horses, carrying…swords.
Well "sabres," to be exact. That's the kind of curved sword a cavalrymen would have carried. Here's a picture of a sabre.

Flashed as they turned in air

The image of these flashing swords makes us think of Medieval knights fighting.
At this point, during the Crimean War, fighting with swords was already becoming obsolete. Can you imagine charging on horseback with a sword toward an enemy with guns and cannon? Focusing on these old-fashioned sabres is another way to point out the desperate heroism of the Light Brigade, and also a way to connect them to English warriors of the past.

Sab'ring the gunners there,

It turns out that the Light Brigade has some luck. They reach the guns and stab the men who are operating them.
It's a vivid image, isn't it? You can just imagine those swords slicing, chopping, and stabbing. This is serious, brutal warfare.

Charging an army, while
All the world wondered.

The doomed bravery of these 600 guys "charging an army" jumps out at us again.
The speaker imagines that "all the world wondered" at this charge. That line needs a little unpacking. In this case, to "wonder" means to be amazed by something. That means that the people who "wondered" were filled with awe as they watched the battle.
What does the speaker mean by "all the world"? Well, Tennyson wrote this poem because he read about the battle in the newspaper. The men of the Light Brigade are world famous.
Now it's not just the people on the battlefield who are amazed by their bravery, but "all the world."

Plunged in the battery-smoke
Right thro' the line they broke;

The Light Brigade is still kicking butt. They move right through the smoke that's coming from the "battery" (that's a group of cannon).
They even break through the line. That's a major moment in a battle at this time. Back in the day, soldiers would line up on a field and shoot or run or ride at each other. For an attack (a "charge") like this to succeed, the soldiers need to get through the enemy line in order to do damage. Think of this like a really brutal game of capture the flag.

Cossack and Russian
Reeled from the sabre stroke
Shattered and sundered.

This is actually the first time we hear about who, exactly, the Light Brigade is attacking. In these lines, they are slicing "Cossack and Russian" soldiers with their swords.
This poem is describing the Crimean War, when Britain and its allies were fighting the Russian Empire. The Cossacks were famously fierce soldiers allied with the Russian Empire.
The soldiers of the Light Brigade are so effective that these enemies are "shattered" and "sundered" (which means broken in two).

Then they rode back, but not
Not the six hundred.

This is a key moment in the poem. The main action so far, the charge, has gone as far as it can. Now the soldiers have to turn back where they came from.
Not all of them though. Some have died. The simple phrase "Not the six hundred" is our first hint of the terrible casualties the Light Brigade has suffered.
The poem has been a little grim, but now it starts to become really mournful, like it was meant for a funeral.