
It’s pretty obvious that the Red Alloy is Captain Falcon, same stance of punch. Simple.
Some thing the Blue Alloy is Zero Suit Samus because of the ponytail, but this isn’t so. The ponytail means nothing, as the spiked shoulder pads mean nothing for the Falcon Alloy and the Horns on the Yellow Alloy don’t mean Meteos either.
Instead of Zero Suit Samus others thought of Zelda (she was used for Multi-Man Melee, so why not Multi-Man Brawl?) but I thought Peach.

Here I noticed the arms. As you can see, Zelda’s left arm extends almost flat out where Peach and the Blue Alloy both crop their left arm up at almost a 90 degree angle. Match.

And here you we see that the legs of the Blue Alloy and Zelda don’t match up. Zelda’s legs bend inwards toward her rear and the Blue Alloy extends her legs out inform of her, Matching Peach’s legs and feet placement.
Just because it has Horns doesn’t mean he is Meteos, same with the ponytail.

(Because Lucas is a Ness clone, by movement of walking, body and some moves I will be using Ness as an image reference)
I first thought of Ness/Lucas when I saw Yellow Alloy because of the arm placement to the torso and feet.
See how his left arm is tucked back and right pushed forward like the Yellow Alloy. It looks the same but its not. It’s actually Mario. The glowing balls for hands threw me off, the Alloy is actually holding a fist and angled right arm like Mario as he keeps his right arm tucked back extended a tiny big to the side.

Also, we can see here that Mario falls much more similarly to the Yellow Alloy than Ness does, with the legs extended downward, not to the sides, and feet match respectively to Mario’s.

As for getting hit my Mario’s side Smash, Ness’s head is tucked way too far down to match the Yellow Alloy while Mario’s frame matches the extended arms and bent legs with head in proper place.
And as for the Green Alloy I see it as Kirby. I know its kind of a “duh” but Ill explain anyway.

Seen here Kirby’s legs match the Green Alloy’s by the angle in which they are extended away from each other and his turned body.

Biggest proof I see is when Kirby and Green Alloy are hit by Mario’s side Smash. Both are bent in similar fashion and feet, one bent up one bent down.

Here we see Kirby standing on his toes (or whatever he has) tilting forward to get a better look at something or other, in game mode and the Subspace Emissary, and his form seems to Match that of Green Alloy’s stance in the opening picture.
So from these visuals and explanations, these are my deductions.
Red Alloy: Captain Falcon
Blue Alloy: Peach
Yellow Alloy: Mario
Green Alloy: Kirby