I think this is a binomial distribution problem but I just don't know how to solve it. It gets a bit hard when I have to decide which is x, n or p

If it's not a binomial distribution problem, please explain me in both cases. Thank you!

Problem:

The median annual income for heads of household in a certain city is $18,000. Four such heads of households are randomly selected for inclusion in an opinion poll. Find the probability distribution of X, the number (out of the four) who have annual incomes below $18,000.

b) Is it unusual to see all four below $18,000 in this type of poll? (What is the probability of this event?)