Super Saiyan 4 is the 4th evolution of Saiyans. It has been achieved by many warriors in Universe 13 and a few in Universe 14.
Super Saiyan 4 is typically the next form achieved for Saiyans who don't use God Ki to obtain their new forms. It is a very powerful form.
List of Warriors who have achieved Super Saiyan 4
Vex has achieved Legendary Super Saiyan 4, but that is a different form from typical Super Saiyan 4, and does not include him in this list.
History of Super Saiyan 4
The first person to have achieved Super Saiyan 4 was Kanji, in a desperate fight against Kurzin when all seemed lost and with Kurzin threatening the lives of his loved ones, Kanji awoke into a Super Saiyan 4.
Eventually after Kanji defeated Kurzin, Resurken, was in a fierce battle with the only two left standing being himself and Ryu. Ryu decided the only way to defeat the enemy was to sacrifice himself in one giant explosion. After the enemy still stood after Ryu's explosion, Resurken exploded into a burst of fury, transforming into a Super Saiyan 4, defeating the enemy.
Ryu was the next warrior to achieve the form, having gone through the mental stress of the thought of his own son and rival surpassing him to heights he may never achieve, sent him into a rage where he found himself awoken as a Super Saiyan 4.
Choken, being the first warrior in Universe 14 to unlock the potential of the form, used it in the battle against the ancients. Seeing how everyone around him had fallen and him always being too powerless to change anything awoke into the next level of Super Saiyan.
Chiyo had achieved the form generations later in an ultimate battle for his home planet. After being knocked down in just a few punches, finding himself to be the weakest member of his friends, he awoke into a Super Saiyan 4, surpassing everyone.
Rinder, similar to Ryu and Resurken, refused to be left in the dust, and while training on another planet, became the next Super Saiyan 4.