My hair is majority 4b with a dash of 4a and a sprinkle of 3c. I occasionally define my 4b curls when I want to sport a different look. It's nice to be able to try out different styles with my hair. But to be honest it's a bit time consuming so I prefer to wear my hair in other styles.
Defined 4b hair does not look like 3b hair or 3c hair. My 3c curls are a lot larger and they hang down more. So, I don't think that the majority of people with 4b hair define their curls to make it look like another hair type.