If the chickens are free range and run around my garden daily, they only go in the coop to sleep, or the run to eat/drink, do I still need to follow these guidelines?
Partly yes and partly no.
Roost space: yes, they need that much space, because that is where they are sleeping. So 4 feet of roost would be the recommended amount for your 4 chickens to sleep on. That gives them enough room to spread a few inches apart in hot weather, or they can snuggle tight together in cold weather.
Nestboxes: yes, because that is where they will lay their eggs. Your prefab coop has more than 1 nestbox, so it's fine for 4 hens.
Ventilation: probably yes, because they need the ventilation when they sleep. They need the ventilation so they are not too hot as they sleep, but they also need it (even in winter) because of the ammonia from their droppings and the moisture from their breath. Chickens poop a LOT while they sleep, so it really is a big point to keep in mind.
Floor space: if the chickens are NEVER shut in while the sun is up, the floor space probably doesn't matter much. Those guidelines assume the chickens are sometimes shut up in their coop, and are often shut up in their coop + run.
Reasons that chickens might be shut in:
--at night, to keep them safe from predators. This means they are stuck inside until you come open their door in the morning.
--bad weather, either because you close them in, or because they put themselves in to avoid the snow, rain, wind, etc.
--you get sick, or your schedule changes, or you go away on vacation. Any of these can interfere with opening/closing doors for the chickens.
--the chickens start flying out of your backyard, a neighbor complains, and you have to shut them in the coop & run while you figure out a long-term solution
--a predator comes into your yard in the daytime and scares (or kills) your chickens, so you shut any remaining chickens safely in their run while you decide what to do next.
--the chicks poop on your porch, scratch up your flowers, and generally make a nuisance of themselves, so you shut them in their run while you construct a bigger run.
--in the past few months, quite a few keepers have shut their chickens into covered pens to try to protect them from Avian Influenza that can be spread by wild birds.
The usual guidelines are written to allow the chickens to be shut in, for extended lengths of time if needed, because there are so many common situations when that is needed.
Not full size yet, We only have 4 chickens. Two are 3 months old and the other two are only 1 month.
By the time they are about 4 months old, most chickens are close to their adult size. They will continue to get heavier, but not much taller or longer. A common guideline is to provide adult amounts of space by the time they are 8 weeks old (2 months.)