She has been thoroughly trained to go in diapers by this point, and has probably lost touch with her bodily functions. You can't expect her to just switch to panties without at least a few days of mess.
To her, something ON means that it's OK for her to use it to pee or poop in. That's the lesson that you have inadvertently taught her for the past four years. She is not being stubborn, and it's not that she doesn't GET it. You are changing up the rules on her all of a sudden, and it will take her a little time to adjust.
First of all, ditch the diapers and pullups completely. You aren't doing her any favors there, it's inconsistent and confusing.
Next, how often does she pee and poop at various times of day? You should have some idea, if not, then pay close attention to her and find out. Normally kids pee more often in the morning than they do in the afternoon.
If she has panties on at home, then set the timer for whatever her frequency is, be it 30 minutes, an hour, whatever. Tell her "Time to go potty!" whenever it goes off, and then HELP her go potty. Pull her panties down for her and sit her on the potty or toilet, wipe her, and pull her panties up when she's done.
Over a few days or weeks (depending on how she responds), you can have her take on these tasks for herself one at a time, but to expect her to figure this all out on her own all at once is a bit much. Practice, practice, practice, and keep up a running commentary. Communication is very important.
"Oh, time to go potty! First we go to the toilet, then we pull panties down, then we pee/poop! When we're all done, we wipe with toilet paper, pull panties back up, and back to playing...hooray! You kept yourself clean and dry!"
Repeat this every single time you take her potty. It will help her get into the routine, and make her feel like you are on her team and ready to help her with this, instead of making her feel punished for not figuring it out all on her own.
Expect some messes at first. It's NORMAL as she figures things out. It will pass. If she has an accident, tell her each time, "Oh, you peed in your pants, that must feel wet and icky. Next time you tell mommy when you have to pee, and I'll help you get to the potty in time. Now let's get you clean and dry again. It feels MUCH better to be clean and dry, doesn't it?"
When you take her to the bathroom, stay there with her until she pees. Or you might try putting a little potty in her play area. It's often very difficult for little kids to tear themselves away from play to go to the bathroom, but if a potty is there close at hand as a reminder, it if often much easier to self-initiate.
If you insist on her using the bathroom, then it often helps to provide certain special bathroom-only toys for her to play with while she is waiting to pee/poop. A drink of water can help get things going as well...it "primes the pump", so to speak.
Hope that helps!