From Dragon Ball Wiki - Legendary Super Sayan 4
Broly's Super Saiyan 4 form is largely similar to Goku's Super Saiyan
4 form, although the main difference is his having a significantly
built frame and blank eyes (similar to his Legendary Super Saiyan
state). It also has a yellow aura when in use, and is one of the few
Legendary variants to not have Broly possess emerald hair.
The Legendary Super Saiyan 4 form, like Legendary Super Saiyan 3 and
Legendary Great Ape, is only seen in video games. Broly is shown
attaining this form thanks to assistance from Black Smoke Shenron, who
uses his shadows to transform Broly into a Super Saiyan 4 in the JM7
trailer for Dragon Ball Heroes, and then proceeded to fight and nearly
kill Super Saiyan 4 Goku and Vegeta, and Super Saiyan 3 Beat but was
struggling against Super Saiyan 2 Note before Super Pikkon arrived.
Broly Super Saiyan 4 is different from other Super Saiyan 4 transformations as it was attained through the negative energy from the Cracked Dragon Balls and Black Smoke Shenron.
The red aura is present at Super Sayan 4 Full Power which is more
powerful than the Super Saiyan 4 form, seemingly a combination of the
former and Legendary Super Saiyan. Broly attained this form around the
Assault on the Hell Gate Saga and used it to defeat the Time Breaker