I also wanted to let you know, and this I read in the book "Curly Like Me". Biracial hair usually is very fragile and thin, much more than "black" or straight hair , even though it may appear very thick. Any kind of chemical will ruin the hair and it will start breaking and stop growing. The hair will become stiff and will stand in several directions.
Originally Posted by Eshansmommy

I haven't read the book, but I thought 4a/4b (although mixed people can have this hair type, but for the sake of my point people always assume mixed people only have 3b/3c hair) was the most fragile type of hair. Because curly hair is fragile, so it would make sense why its the most fragile, since its the curliest.