Vegito is a "Dragonball Z" character created by combining Goku and Vegeta. The SSJ4 is a power ranking that denotes a base unit, Super Saiyan (SS) and levels above this base level (J4). However, Vegito is never illustrated with this level of power ranking.
The merging of two "Dragonball Z" characters is done through the fusion dance. If the timing of this dance is off, one of the characters ends up as either fat or skinny, showing an imbalance of power. The timing of this fusion dance lasts for 30 minutes as the two characters automatically separate and become two characters again.