Well the poor little lady! Usually when people talk about hair loss they mean thinning out or little bald patches but that's very dramatic. I can't say I've ever seen anything like that before. Presumably the top of her is looking completely fine and not shedding?
It's definitely going to have to be a vet appointment. In your shoes I would also take her cage mate just in case it's something contagious. Many owners take the companion along to the vet anyway to keep the patient company. I use a cat box rather than a small pet carrier as it's big enough for two.
You say she's about 4. Have you had her for a few years or is she a new arrival to you?
What bedding is she kept on and has anything changed recently in the cage - perhaps a change of washing liquid for washing fleeces or fabric tunnels etc.
Her skin in the pics is actually looking pretty normal - not red, inflamed or sore in any way as far as I can see. This is good at least.
Have you noticed her grooming or nibbling her tummy in any way? Or if you have another piggy have they been grooming her?
Finally, when did you notice this? I get the impression from your post that you turned her over quite recently and just thought "OH NO!" Now that's fair enough - we don't routinely turn them over as they don't like it, and personally I'm not a piggy cuddler so I don't know if I'd spot anything happening myself, but...
Did someone point it out to you that she might have some hair loss underneath that prompted you to look?
Did you seen any of the hair in the enclosure? Maybe in clumps or just general shedding over time? Because that's a lot of hair missing. Or was there no sign...
The reason I ask is because one thing we have to consider is whether a housemate, boyfriend, brother etc has actually clippered your poor girl as a prank. If a person said "Hey have you noticed..." I'd be terribly shocked of course, but also a bit suspicious (and would check the bins for fur) (and her nipples for damage).
It's probably not this - please do see the vet - it's important she's checked out as some of the causes of hair loss can cause discomfort or distress for piggy. Tell the vet as much of her history as you can, even if you don't think it's especially relevant. It can help to make some notes before you go. Until then all I can suggest is you give her soft surfaces to lie on and put her hay in front of them because if I had to sleep on hay on a bare belly I'd be itchy.
Please do keep us updated x