The general rule of thumb and healthiest for your chickens is 4 sq. ft. per Standard size chicken inside space and 3 sq. ft. per Bantam inside space. If they don't have access to an outside run all day then I'd make it much larger than that even. If they do have access to outside then that would suffice.
I don't know where you live but the real issues arise if you ever have to lock up your chickens for even a day due to a snowstorm or rainstorm, etc. When chickens are crowded they tend to featherpeck each other and get stressed over the tight quarters. (I learned this the hard way - through personal experience.)
You need to take into consideration that your waterer and feeder and possibly your nest boxes (depending on your design) are taking up floor space so you can't count that space. That's something that many people forget to think about.
Also, make sure your roost(s) are higher than your nest boxes or you may end up with chickens sleeping in their nest boxes and then you get poopy eggs. (Also learned this from personal experience..
And, as already mentioned. Build much, much bigger than you really need because you will want more chickens! You can't help it, you're part of BYC and we are true enablers.
Roosts are where chickens sleep at night. You should allow 10" for each bird, so 40" of roost space for your 4 chickens. They will, however, often crush against each other and only use 1/2 that space... but offer it, just in case. Most people put their roosts toward the top of the hen house. Chickens love to get higher when they sleep, so they'll typically get up to the highest point they can. Thus, why you want the roosts higher than the nest boxes.
Nest boxes are where the chickens lay their eggs. They are generally 12x12 in size (depending on the breed you have), some people use 5 gallons buckets with the lid off and cut in half then put back on, some people use milk crates. Usually you would put the nest boxes a few inches or foot off the ground. Some people set them on the ground and some people put them 2 ft. off the floor. Again, what works for you in your situation. It's nice to have a "perch" just outside the nest boxes (if they are in the air) so that the chickens can jump up onto the perch and then walk into the box.
One nest box for every 4 chickens is fine. So, you could have just one. I'd suggest having two, though, if you have the space just so there's a 2nd choice if one hen is using the other nest box. Then again, I have 8 nest boxes and my chickens almost always use the same three!