Thanks for the advice guys. I don't feel like I need an actual trainer to help me because I do have a lot of horse experience and I have trained my own colts and helped friends with their horse problems. I will eventually figure out what works for him, I was just curious to see how others have went about this problem.
I'm sorry, but I see people who make this statement all the time on here and I just want to ask: Well then if you know everything and can't possibly learn anything from anyone else, why are you on here asking questions?
A trainer (a reputable one, that is) is always
going to teach you something you didn't know before. I didn't have my first official lesson until I was 24 years old. I've been riding since the age of 2. I trained horses for people in high school. And boy did I love the lessons! So many things a second set of eyes on the ground can pick up. I make sure to take a least a couple lessons every single year in different things, because it can only make me a better horseperson.
Anyway, ranting aside..... Chances are this horse has never been asked to respect people. So you have to treat him as if he has never been handled before. You'll have to start completely from scratch. And I wouldn't expect your little sister to be able to handle him for a long time (months). This 4-yr-old has learned some serious bad habits and it is going to take a long time to earn his respect and trust. Ground work, ground work, ground work! Timing is so very critical, as is body language. If you leave the pressure on too long, he'll learn to brace, resist, rear, etc. If you do not leave the pressure on long enough, he'll learn he can get away with anything.
I would not tie him at this point, because he doesn't know how to respond to it. Start with the basics. Put a tiny amount of pressure on the lead rope, let's say we are asking him to walk forward. He might resist, or "flip out" as you state. Whatever you do, do NOT release that pressure until the very instant he gives to it. Don't pull harder though; just hold steady. If he moves, you move with him so you stay the exact same distance from him. But you've got to be ready to give to him when he gives to you. This is where timing is crucial and would be much better explained if you took a few lessons from a trainer in person so you could be shown.
You also need to immediately correct him when he crowds you when leading. It's very dangerous to have a horse walk all over you and not respect your space. I myself like to carry a whip when doing ground work, because it makes my arm longer.
Disengaging the hindquarters is going to be an important lesson for this horse. Yes, we want to control other parts of his body too, but if we can control the hindquarters, we can control a lot. I personally like Clinton Anderson's methods because he explains things well, including timing. Yes, this is a trailer loading video, but trailer loading issues ARE a ground work issue. So he does ground work with the horse. Watch closely to his body language, when he releases pressure, how he gets the horse to disengage the hindquarters and respect his space.
This is not a small task you've brought upon yourself with an almost full-grown horse who has bad habit engrained into him. You are going to have to handle him every single day, and you are going to have to be very consistent.
There's tons more I could say, but I'm going to stop for the time being. Because again, a trainer is going to help you more than anything at this point in this situation.