Super Saiyan 4 is the fourth form of Super Saiyan a Saiyan can take, directly following Super Saiyan 3. In a way, Super Saiyan 4 isn't a standard form like Super Saiyan 1, 2, and 3, since Super Saiyan 4 is the stage after Golden Oozaru and not Super Saiyan 3. Unlike Super Saiyan 3 does not use alot of energy quickly, so it can be easily held. It's power is about as strong as Syn Shenron.
A Super Saiyan 4's hair is like Super Saiyan 3's but not as long. It also changes back to the normal color. They also gain back their eyebrows. Aside from the hair, they gain red fur on their chest and new pants.
The original user of the Super Saiyan 4. He is the first to use it, the first to master it, and generally the first to obtain new forms of each stage of Super Saiyan. It is safe to say that Goku is the master of the Super Saiyan.
Vegeta was the fourth Saiyan to achieve Super Saiyan 3.
Neither Gohan nor Goten can use Super Saiyan 4. Though since they have both mastered Super Saiyan 5, after fusing they can use Super Saiyan 4.
Bardock learned Super Saiyan 4 after watching his son use it against Bebi.