Usually at a quinceañera they showcase a dance they have been practicing so the answer, waltz, is the most logical choice
1.van venir. Vienes
2. voy
3. van
It should be this, I grew up with spanish as my first language and this is how we say it. I hope its right because we sometimes tend to use slang and sometimes we don't.
It might be different by the Spanish-speaking country's standards. Maybe you might be confused by what the doctor is saying because you don't speak Spanish well. Maybe the procedure/checkup you're having is different from what you are used in the country you lived at before you visited this country.
4singular in spanish is cuatro.