"Most of the polyethylene ive found is 4 mil, is that thick enough?"

I use 4 mil for garden cloches(greenhouses) and it holds up fine for a couple years of continuous use. They will last longer if you get the much more expensive UV resistant plastic.

I typically make these long hoop houses out of Frost King rolls of plastic. The rolls are 10' wide and 25 feet long and cost about $7 in Seattle.