Let's clear up the story behind those four arms, since it's actually quite interesting. The current form you're seeing isn't her original body at all, but rather a specially crafted doll (this becomes really important later in her questline). Her original form, which you can actually find at the divine tower of liurnia, only had two arms like any other person. The interesting part is that she chose this four-armed appearance specifically because it's modeled after her mentor, a mysterious snow witch who taught her secret sorceries.

The whole doll body situation happened because ranni needed to break free from some rather complicated divine influence (keeping it simple here, but there's quite a story behind it). Now, while we're pretty sure about the doll being based on her mentor's appearance, it's worth noting that whether the snow witch actually had four arms herself is more of an educated guess than confirmed fact. The single head though, that's kept deliberately human-like to maintain some connection to her original form.

Maybe interesting: if you look closely at the marionette enemies scattered around, you'll notice they also sport four arms - might be some connection there with the whole puppet theme going on. And just between us, keeping track of where those four arms are during her questline can help you spot some pretty clever story details that are easy to miss otherwise.