I’m coming to you. This could be used to clarify a situation. There might be confusion as to whether or not the other person was to come to you or not. Person 1: You want me to come to your house at 5 pm, correct? Person 2: No, I’m coming to you. I’ll be at your house at 5 pm.

1. Right now you are on you way to the airport, to emphasize that you are already in motion. 2. You could say this either when you are about to head for the airport or when you are on your way.

"I'm on my way to the airport" would be used when you are literally in the car and reporting to someone about it in real time, while in the process of going. "I'm going to the airport" could be said for any time condition. You could say, "I am going to the airport now, or tomorrow, or next week." But we would never say, "I am on my way to the airport next week." "On my way means" right now this instant. If someone is late they would call and clarify, "I am on my way." If they said "I'm coming" this would be a bit rude and not clarify that they are moving. "I'm coming" could mean they are still at home and eventually will leave." But "I am on my way" means they are rapidly approaching and will be there soon.

The meaning conveyed in all of the sentences is the same. There isn’t really any difference. Although thinking about it more, I normally only use I’m on my way to you. Or in text abbreviated “omw”. I use “okay I’m coming to you” if I am in a crowd trying to get to another person who I want to stay still. This isn’t any hard and fast rule, it just when I use them thinking about my life. All are interchangeable

No difference; #3 ‘I’m on my way’ is sufficient, #4 ‘I’m coming’.