My 4 year olds think listening to me is optional

It’s driving me mad. I have put so much time and effort into positive disciplining and not spanking. I will tell them not to do something and they’ll do it anyway immediately after and so after awhile I’ll get frustrated and say “I just told you not to do that. Why did you do it?” And they’ll flat out say “because I wanted to.” Not being bratty, that is just their honest answer. They say it so innocently. Or also they started this thing where they SCREAM when they’re mad. They freeze, scream out of anger this ear-piercing scream and legit stand there and shake, and then cry til they’re red in the face. Today toddler A accidentally hit into toddler B and scratched her arm, so toddler B whipped around, screamed and shaked and cried. Cried for over 5 minutes about an accidental microscopic scrape. Same thing if toddler A wants a toy toddler B has, one time I was trying to get them to split some playdough in half so they had equal amounts (since they were bickering about it not being fair who had what blah blah blah) and toddler A didn’t want to because she wanted the bigger wad of playdough. So she froze, screamed out of anger and turned purpley red. I’ll tell them to take a deep breath to calm down instead of screaming/shaking, have some alone time, and then come back out and talk so we can fix the problem. Legit 2 minutes later they’re screaming and shaking again about something new. It’s maddening! What am I doing wrong? Is this age appropriate or am I failing as a mom in this department? It’s making me want to run away and live on my own some days.