Ahhh to be a new C owner again. It was fun discovering all of these things. It's even better with the C 4.
I have the 2012 Prius Four. I took the armrest cover off, and while I'm no automobile expert, I am a licensed mechanical engineer and I can't figure out how it might slide forward. Maybe the sliding armrest was just an option in 2012?
That is not the PC4 center armrest. Looks like someone swapped yours out.... or do you really have the Prius 4 and not the Prius C?