I'm surprised you say the coop is really small, you should have seen the size of the prefabricated ones that say they can hold 10+ chickens.
Those prefabs are a joke, shouldn't be used as any kind of standard to compare against at all. We call them dollhouses on BYC, because that's what they are good for - dolls, not chickens
Or maybe as a brooder, or isolation/quarantine unit etc. but not as an actual coop.
The problem with small coops like yours (yes it's small, maybe not as tiny as some prefabs but still small) is that it's hard to ventilate them well, because they don't have enough height. 4x4 on the floor could be fine, if the whole structure was taller, so you'd have enough vertical space for a roost plus ventilation that's high enough not to blow at the chickens directly. In your case, you could probably get away with less height if you stick with silkies, because they don't roost high, if they roost at all. Their fluffy feathers are cute, but defective by their very nature, and can't properly do the job feathers evolved to do - give birds flight (or help with jumping, as with flightless birds), and insulate them from hot/cold/moisture. Humans like to breed defective animals if they look cute, and propagate their flaws (just look at some dog breeds, that can't breathe or walk properly...) So anyway, that's why silkies don't like to roost high, or at all - they can't jump the way normal chickens can. And they can't keep warm the way normal chickens can, so they huddle for warmth (normal chickens don't huddle for warmth once fully grown - their feathers insulate them and don't let heat escape, so they can't heat up their buddy with their own body heat). If the roost is low enough and wide enough, they might take to it. It's better to help them out with the setup, and maybe put them up on the roost for a while, to teach them to roost. Some can be okay sleeping on the floor, but they run the risk of sleeping in their own poop and messing themselves up, which can create other problems.
Anyway, back to ventilation. Being "airy" doesn't count as ventilation. The space is still pretty small, and, regardless, you need an active exchange of air with the outside, to get fresh air in. In addition to moisture, the air in the coop is also laden with ammonia vapor from their poop. That's harmful to their respiratory systems. Theirs are more sensitive than ours, so what might seem and smell fine to you, may not be so fine to them. If they sleep on the floor or very low, that's even worse, because they are closer to the poop (if not rolling in it). You need the moist, poopy air to be able to vent out and be replaced by clean, fresh, dry air from outside. So the coop definitely needs a vent. I'd start with the existing window, dual-purposing it to act as a vent at the top and for light at the bottom (covered with something transparent), and not add any large fowl breeds to the silkies. The larger, normally-feathered birds will want to roost high and will be all over that thick bar, pooping into the feed and water. And the more birds you add, the more ventilation you'll need, because they'll poop and breathe more...
Water inside vs. outside is a personal preference as well as dependent on your setup and climate. And whether you have rodents nearby. I don't like to have food outside, day or night, because I don't want to attract rats, squirrels, wild birds and everybody else who'd like a free meal. They are usually afraid to go into an enclosed space, so even with the pop door open during the day, they tend to not venture inside. And I like to keep my water outside, to avoid any spills in the coop or chickens stepping in it or messing it up with pine shavings. They have an auto door that opens at sunrise, and can go out and drink all they want. They don't eat or drink at night, so they don't need either in the coop, I only keep my feeder in the coop to keep it safe from other animals. The only time I've brought water inside the coop is during very heavy snowstorms, when the run cover can't keep all the snow out and I can't keep up with shoveling, and the chickens don't want to go out. But that's really only like once or twice per year, and not something you need to worry about over there
