Super Saiyan-Jin Level 5?icpg1252\uc1 \
I know that this form does not exist. I made it up because I feel that it could've been accomplished had the writers felt like doing more episodes of Dragonball. This last transformation would've been an addition to SSJ 4.  Like usual, it would've taken much training and time to perfect this transformation.  A burst of anger would've pushed the saiyan into this form.  This form would probably be the final form of saiyan-jin.  In this form a super saiyan would be unbelievably strong.
Advantage:
This final form of super saiyan would push the saiyans powers to their limit.  The super saiyan in this form would pretty much be unstoppable. 
Disadvantage:
This form would likely use much energy.  The SSJ 5 would probably only last for 5-10 minutes before turning back into a normal saiyan.  However the SSJ 5 wouldn't even need this much time to defeat his enemy :)
Interesting Facts:
  Just as Coola, Freeza's brother, has five transformations, so should a saiyan.
This would be the last transformation, even for Goku.
In this form the SSJ 5 would probably be very similar to SSJ 3, except the hair would be a different color (probably the same color as Gogeta level 4's hair).  The eyes would either be green like usual, or a freaky yellow, which makes them look immortal.
Who Can Reach This Form:
 I didn't think that through, probably only Goku.
      (pictures of SSJ 5 Goku and Gotenks)