Thank you for the advice and useful info.
Largely, I thought to partition the drive, because I don't/didn't really know what I should I do to provide her with the extra storage space she needs, whilst preserving a chunk of it for my use.
The other 4TB of the drive I was intending to use as back-up drive for my data from the Mac, which I guess was my biggest thought in why I should partition the drive, therefore dedicating a set amount of space to back-ups from the Mac and set amount of space for her iPad data.
An issue I hadn't considered though and you have brought it to my attention is her photo library, which is where most of the storage on her device is being used up. You mention I can transfer the photos to the home network drive, but I can I transfer the library? Its largely just reference images for her art from the internet that she saves and holds onto for future reference.
I'm not sure if the drive or drive bay I'm using is classified/considered as a Network storage device (I don't believe they are). Its just a simple 5 Bay enclosure from IcyBox (which I'm pretty sure I've saw sold under other brand names too) wired directly into the Mac, and a Seagate Barracuda. I was just intending to use the File/Media sharing option on the Mac to add it to the home network and letting her access the drive that way.
We're not worried about access to the drive (at least for now) outside the home network, as she is disabled and is pretty much house bound, so the likelihood of needing to access the data from anyplace else is limited, as the iPad and her are pretty much always at home.