Dragonball Comedy
Description
This is the final form we see and is only seen in Dragonball GT (so we won't see it in England for a long time yet). This form is completly different from all the other super saiyan forms. For a start, in level 4, a super saiyan no longer has golden hair, they gain shoulder length black hair. Their eyes become red and also gain red fur across the majority of their body and their tail grows back. As well as this, a saiyan wears these funny looking MC Hammer style gold trousers with a sash around the waist. A super saiyans power is far beyond that of a level 3 super saiyan as this form is the hardest to acheive. This is seen in the fact that only Goku acheives it the proper way. Vegeta needs the assistance of the Brute Ray to reach this level. A level 4 Super Saiyan becomes a level 4 Super Saiyan by first becoming a golden oozaru.

The Alternative Theory
After reaching the maximum Nazi superman stage in level 3 Super Saiyan, a saiyan then tries to achieve Nazi god status by turning into thier "Fuhrer" Adolf Hitler. This is clearly seen in the saiyan level 4 stage. The saiyan is already transforming into the Nazi god as seen by the black hair. The gold trousers are part way to transforming into Nazi combat trousers. Unbeknownst to most people, there is a Super Saiyan Level 5 which is almost impossible to reach. Only one person in existence reached it and that name was dropped into the mighty Pot Noodle of time and never recovered. Basically in Super Saiyan level 5, the Nazi god transformation is complete and the saiyan gains a side parting and a crap moustache. However, the nazi god status has its huge power for a very, very VERY short period of time making it effectively a very weak status.