We had a large fenced in area around our chickens, about 4 feet high if I recall correctly, and it kept the chickens in. The fencing was cheap wire fence from Home Depot, kept in place with metal stakes in the ground. We kept the coop in the same place and shifted the fenced area between the right and left sides as the grass got eaten up.

I'd recommend getting a compost tumbler. You can rake out the coop once a week and throw all the straw and chicken shit in the tumbler, along with kitchen scraps. It makes awesome soil in a short time (a few weeks, again if memory serves).

4 chickens is a great number. If you go to 10 you'll have more eggs than you can eat and enough to give to friends. I'd get a mix of chickens based on egg color. It is a lot of fun to take your child out every morning to gather a mix of eggs in white, brown, blue, green, pink, and white with brown speckles. It's like Easter every morning.

Advice: do not have 75 chickens. This is exactly the wrong number of chickens to have. Enough to create serious work. Not enough to create money from egg sales. 4-10 chickens or over 300.