Welcome to BYC
It's almost impossible to sex coturnix chicks until they are feathered. I have between 40-60% roosters every hatch so you surely want extras. Processing a quail only takes about 3-5 minutes so if you have extra roosters it's easy enough to solve. There are videos on youtube that will help guide you through the slaughter and cleaning. I would get 8-10 chicks if I was trying to get four females. Extra females people will buy, extra males you will have eat or maybe you can give them away.
Avoid any chicks with poop crusted on their butts, chicks with wings drooping, and limping or slow moving chicks. Look at the parent birds, they should be clean and fully feathered with straight beaks and toes. Hens being feather picked on the back of the head is acceptable to most people (it comes from too few hens per rooster) but can be avoided. Birds being feather picked all over usually means improper diet or crowding. Pens should be clean with no poop build up, feeders and waterers should be clear of poop also. Any problems the breeder has will come home with you so don't be afraid to walk away from a dirty facility or poor quality birds.
Good luck starting your flock!