Background
While working for the Empire as a courier droid R4-D9 was Heavily Damaged during a skirmish with a band of Rebels.
Those very same Rebels fixed and repurposed him, however this has done nothing to curb his rather...outspoken
personality. Despite not having all of his memories because of the damage R4 is still very useful(When he wants to be) and can easily be disguised as one of the Empires droids with his
classification as an R4 unit, thus allowing him to sabotage and cause mayhem wherever he pleases. When not out fighting the Empire you can find R4 either working on repairing a ship or his personal favorite blasting Cantina Jazz, much to the chagrin of his teammates.
And whatever you do don't ask him what happened on geonosis. He doesn't like to talk about it.
Along with working with the Rebels one day he hopes to get his memories back while taking down the Empire and may the force be with anyone who gets in his way. Seriously...He's made grown Wookies cry.
Motivation
Downfall of the Empire.
Restoring memory.