how common is big clitoris among females?

Every female is different in growing in the womb and during puberty.Your hormones were not in balance in development and your result is a larger clitoris.When you get aroused your clitoris gets lot larger than in its relaxed state.

There are quite a few females in this world that purposely use a clitoris pump to get a larger clitoris.Be happy with what you have.

If you are bothered by your size then see a doctor that specializes in hormone therapy.

my clitoris is 4 cm (how common is big clitoris among females?)

Clitoral sizes are type 1 less than 2 cm
Type 2 is 2-4 cm
Type 3 is 4-6 cm
Type 4 is over 6 cm
According to this chart your clitoris is in the lower end of this scale so there are no known numbers of how many in each category.

Thank you ,
but that size it's normal

There are a lot of guys out there that love a bigger clitoris because it gives them more to focus on when giving oral sex just the same as a female who has a big labia and if the guy knows how to please properly then he can give you the best orgasm from just playing with either.

Don't be ashamed of what you have been given because it's a thing of beauty :)