W-A04 is a prototype Arakyd protocol droid that was "lost" in transit during a pirate attack.  Having hid during most of the attack "she" snuck aboard the pirate vessel and hitched a ride to a new system (looting some key prizes off of the pirates on the way).  Getting off during a refueling stop W-A04 invested in the parts to change her appearance and start the life of a free droid.  Small in stature she often goes unnoticed in most situations.