
Welcome to BYC!
There has been a lot of good advice already. I definitely agree that bigger is better, the 4 sq. ft. per chicken is the minimum you want, otherwise you can have problems with them fighting. The more room they have inside on a rainy or snowy day, the less chance of someone getting picked on. You’ll also want room to move around when you are inside cleaning, 4x6 doesn’t give much room to turn around in when you have a shovel full of shavings. I’d say upgrade now to 6x8 or bigger. Also, that gives you room for chicken math. My coop is 10x16, and I have about 5x10 if that walled off for myself to keep the food and supplies. That gives the birds about 10x11 to roam around in, plus their run.
Ventilation is about the most important thing, especially in winter. A closed up coop will get a lot of moisture buildup, which isn’t good for their respiration and can cause frostbite. They can take the cold as long as it is dry air, and no drafts.
Poop boards are about the greatest invention since the coop! It will save a lot of time cleaning, because most of your poop will be on it instead of the floor. I have sand and PDZ on mine, and scoop it with a kitty litter scoop. The PDZ helps dehydrate the poop, and helps a lot with the smell.
Aart will be along soon, I am sure. You’ll get lots of good advice then on the different hights for nesting boxes, roosts, etc.