There's no way your going to build one of those big enough to hold 4 ferementers for $100. Plus, it's going to take up a lot of space, it's inherently messy, and would require a freezer full of gallon jugs to operate. Better off just getting a freezer and a controller and be done with it.
No, what I said was that you could build 4 fermentation chambers for that money, not one that holds 4 fermentors. If you built it well it wouldn't be messy at all, and when you're fermenting ales, the ice only needs to be changed out every other day.
If you want one chamber that can hold 4 fermentors at once, you're going to spend a lot of money. That's just reality. I don't think you can get 4 in a standard fridge (maybe with some mods and only using buckets)- you'd have to go to a pretty massive chest freezer. That said, a nice, upright fridge on craigslist in SWMBO approved condition will probably still cost you less than half a G.
My feeling is this: if you're only two beer drinkers in one house, and both your square footage and your budget is limited, why would you need to have 20 gallons of beer (eight cases) actively fermenting at one time, plus whatever's bottled, conditioning, or on secondary?