Part of the answer depends on your climate and part depends on how you manage them. In most places a 3' x 6' coop will work fine for four hens, but they need additional outdoor space to go with that. Some people can free range their chickens, but most of us worry about predators or the chickens getting into gardens, visiting neighbors, or playing in the road and getting run over. Some have legal restrictions. If you need to contain them outside, I would suggest a minimum run of say 6' x 8' with bigger being better. The more space you can provide them, the happier they are, the less likely they are to become bored and turn to cannibalism, and the easier it is for you to manage them. One possible problem as far as your time and effort is that if the space is too small, you may have to put some serious effort into managing their poop. Many people get by with the minimum space and it is nice to have "chores" for your young boys to do, but if you provide a bit more space than the minimum, your life is easier.
If you live in an area where they cannot get outside for long periods of time due to climate, you might want to cover part of the run to keep snow out of it and to block the wind. Some of mine will go wading through 9" of snow to check out my compost heap in case I've thrown something interesting there and some will go out and forage in three or four inches of snow if the weeds and grass is sticking up through the snow, but most do not like the snow at all and they really hate wind. But if they can get out most days, your 3'x6' should be OK for 4 hens. Mine wander around in single digits weather, so cold doesn't bother them very much, but wind and snow does.
The rule of thumb is one nest box for every 4 hens, but I suggest you provide two. They will all probably lay in one anyway, but I find that an extra one can come in handy, say if you have a broody. Yesterday I had three crowded into one nest box while another one was open, but sometimes a hen can be very protective of that nest box. Nest boxes are easy anyway. Here is a thread that gives you an idea of the wide variety of things that people do for nest boxes.
Nesting Boxes
https://www.backyardchickens.com/forum/viewtopic.php?id=41108
2 x 4's work great for roosts. Just round off the corners so their feet are more comfortable when they wrap around, but mainly to remove any splinters. It does not take much sanding to take care of that. For 4 hens you can get by with one three foot roost, but I would be tempted to add a second roost. They will probably all crowd together on the end of one roost to sleep, but I like to give them a bit more room to get up there. Also, sometimes you get a real bully that does not like to be crowded on the roosts. I like to provide a bit more space so the other one(s) can get away from the bully if they want to. I find it keeps them from roosting in nest boxes or other strange places.
You'll notice I use "like" a lot. That indicates a lot of this is personal preference.
Another issue for you is how high to make the roosts. I suggest as low as possible but to make them so they are noticeably higher than anything you do not want them to roost on. I base it on the nest boxes. You can put your nest boxes on the ground or put them up higher. Again, pure personal preference. Some of the things to consider with that. On the ground is real easy to do, but some people find that the nest boxes on the ground tend to get dirtier. If you have a bad back, you may not want to bend over to gather the eggs. If you raise them, raise them enough over the bedding that you can get under there. Chickens tend to scratch a lot, so they tend to scratch bedding where it builds up under nest boxes. You may need to rake it out occasionally. If there is not enough space for a chicken to get under there, it becomes a great place for Mommy Mouse to raise a family. It can become a place a hen thinks is a great place to lay an egg, so you need to be able to see under there and get under there. As you can see by that thread above, there are many different ways we do it. Don't obsess over what you decide.
Back to height. I find that I need about a 5" to 6" lip at the bottom of the nest box to keep the hens from scratching out the nesting material or eggs that are in there. They often like to scratch and rearrange the nesting material before they lay. Above that lip, I suggest about an 8" high hole for them to get into the nest. I've used a 6" high hole and that works, but again I like a bit more room. That personal preference thing again. Then depending on how you build it, you might need some room for the frame. If you use a bucket or cat little box, something like that, you might want to tilt it back some to help keep them from scratching nesting material or eggs out of the nest. Whatever you do, make the roost noticeably higher than the top of the nesting box since chickens tend to like to roost on the highest thing available.
I suggest making the roost as low as reasonable because chickens have been known to hurt themselves coming down from the roost, especially the larger breeds. It is not that they jam their joints every time they hop down. They spread their wings to come down fairly slowly and all that, and it does not really happen that often, but it is something that can happen. I just think it is good practice to keep the roosts as low as practical.
Another consideration, especially in your coop since it is 3 feet wide. Try to keep the area in front of the roosts clear to give them some room to land. Some chickens can fly reasonably well, but they are not really the most graceful things with wings. They can injure themselves coming down by hitting nest boxes, feeders, waterers, whatever, so try to give them room to land when they come down. That is another reason to keep the roost fairly low. The higher it is, the more room they need to land.
Another reason to keep your roost reasonably low. Regardless of your climate, you need ventilation in the coop, both in the winter and the summer. In the winter you need to get rid of moisture, in the summer heat, and in both seasons ammonia from their droppings. The easiest way to provide a lot of this ventilation is to have vents at the top of your coop. But you do not want the chickens roosting in drafts, especially in winter. So if the roosts are low enough so they are not in cross breezes from those vents, you are in good shape.
I'll give you links to some articles by a lady that lives in Ontario that really knows her stuff. I think they should be required reading for anyone that is starting out.
Pats Big Ol' Ventilation Page
https://www.backyardchickens.com/web/viewblog.php?id=1642-VENTILATION
Pats Cold Coop (winter design) page:
https://www.backyardchickens.com/web/viewblog.php?id=1642-winter-coop-temperatures
Pats Big Ol' Mud Page (fixing muddy runs):
https://www.backyardchickens.com/web/viewblog.php?id=1642-fix-a-muddy-run
I know I have probably made it sound more complicated than it really is. With your existing building, you should do great. Welcome to the adventure.
