Are you planning to get them as adults? If not, you can't get a guarantee of them being hens. Even the best sexing methods only hit about 90% accuracy in chicks that aren't sex-linked or autosexing.
You'll want to build up an emergency buffer before you get them. Enough to pay for medication, a vet visit, or both if something happens. That way, if one gets sick or injured, you can do something about it. You should probably also get some of the non-perishable supplies for a first aid kit, like bandages and cornstarch. Figure out where in your area sells chicken medical supplies, and see if any vets in your area will treat them.
Those prefab coops you'll see for sale are way too small for what they claim they can house. You need at least 4 square feet per bird in the coop, so at least a 4 foot by 4 foot coop for your 4 hens, and as much run space as you can give them.
My advice would be to figure everything out beforehand. How you'll pay for their food, what you'll do if one gets sick or injured, how you'll build the coop and with what, that sort of thing. Once you have all that, you can ask your parents, and show them that you more or less have an idea of what you're doing.