Problem to edit File on sd, sd.seek(), sd.print();

Hello, please help me. So when I write something in file myFile.print(); work fine. after when I want to change content for example need to set cursor on position 8, I use myFile.seek(8); and also work fine but after seek I want to insert text and it not inserted on 8 position it start writing from end of file, I also try myFile.write, can anyone help me?

myFile = SD.open("test.txt", O_READ | O_WRITE | O_CREAT | O_APPEND);

Serial.println("test.txt");
myFile.print("1111\n");
myFile.print("2222\n");
myFile.print("3333\n");
myFile.print("4444\n");
myFile.seek(8);
myFile.print("fffffffffffff\n");