SS 4 is a very odd level as it is very different so some people don't consider it as a SS level at all. In this level the Saiyan has the same colour hair the usually have but a bit longer and slightly different, their body apart from chest is covered in red fur like a monkey, they have green eyes and big muscles. They also have their tail back if they did not all ready have it (they need a tail to turn to this level).
Saiyan's turn to this level by being in SS and then looking at a full moon (full earth in Goku's case) they then go to Ougan Oozaru form (golden monkey form) from this they get angry and turn into things like were-monkeys they then feel angry, sad and in ways joy and then turn to SS 4. From then on they can turn SS 4 straight from normal. Goku transforms by being angry at baby and then seeing his daughter Pan crying which makes him turn to SS4.
The SS 4 transformation is even harder than the SS3 transformation. The main reason it is so different is that it was invented by the people at Bird studios rather than Akira Toriyami. They were originally going to call it something different like monkey level one but couldn't be bothered in the end. I think SS 4 would have been like SS 3 but the hair would be spikier.
The advantages are being incredibly strong but the disadvantages are that it uses up lots of energy (when your energy is used up in this form you go back to a normal Saiyan not even a SS. It takes up so much energy that it makes fusion last for 20 mins rather than 30 mins. It is also very hard transformation first time.