We purchased 4mil polyethylene that was supposed to be 'clear' but it is opaque white. We can not see through it at all. Is this normal? Will this work for one year?
It depends on your environment and how much light is blocked. Basically opaque plastic might even let 70-90% of light true even though you can't see through it. This depends highly on the plastic. If you're in lowlight environment then that would be bad but if you're in environment where you have enough or too much light then this should not cause problems. It all comes down to what kind of plants are you growing and how much sunlight you receive.
EDIT: Usually manufacturer has the data about light passthrough. Photographers light meter will help assess the amount but spectrum is different thing and you just have to hope the plastic cuts everything evenly. Also photographers light meter is adjusted so that it takes into account the light that humans see so it's not good measure on how much plants actually get as these are different spectrums.