Here. Let me save you a lot of money and aggravation.
This whole chicken tractor thing looks great on paper, until you actually get some chickens. The idea of a portable coop, with attached run does not make practical sense.
They are expensive, not big enough, and often not very secure. When I got mine, I went to all the forums and did the math on how much space each hen needs. And I can tell you that I currently have four free-ranging in my back yard (about 1/4 acre fenced-in) and I'm watching the ornery buggers getting into my garden as I type. Chickens are the most curious, active, dissatisfied creatures I've ever had the pleasure to know. They spend at least half of the day foraging and looking for entertainment. Keeping them in one of those little runs borders on cruelty (IMHO).
I came about mine, unexpectedly, and didn't have a lot of time to find a solution. The first thing I did was pay a lot of money for a tractor. The second thing I did was put wheels on it; because the thing is damned heavy! Unless you want them to scratch around in a small square of dirt and shit for the rest of their lives, you need to move the tractor. In my case, I ended up moving it every day (every other if I got lazy). Still, I had a yard full of ankle-breaker holes.
It quickly occurred to me that they needed more space; so I built a portable day-run for them. This helped a lot, and the holes were minimized; as long as I kept it moving.
Ultimately, I ended up turning the tractor into a coop and letting them have the run of the yard. The coop has some issues and isn't perfect; but it did house five hens (when I had five). Funny though, that they all still crammed into the top (old tractor section) to sleep. My leghorns have bent tails from this; but they aren't interested in the new roosts I added to the new section.
Here is a gallery of my original tractor (with modifications), the portable run that I made of PVC and bird fence, and their current digs "Chick Chalet". I also got an an automatic door, otherwise my life would revolve around always being home to let them in and out.
Look on craigslist, and you'll probably find a used coop or tractor for a fraction of what you'd pay new. You may also end up with a full-fledged flock and some information (and horror stories) from the owner. You may walk away realizing that you don't want chickens, after all. They are messy, noisy, they get sick and injured, everything tries to eat them, and they stop laying for months at a time. Yet, they never stop eating...
If you want to build your own portable run, it costs about $40 in materials. I made mine with an opening to fit up against the tractor door, and with a flap door, so that I could keep them contained when it wasn't attached to the tractor (cleaning, etc.). If I were to do it over, I'd make it a bit larger (especially for four hens), and I'd cover the top with shade cloth, instead of bird-netting.