Not enough info. ASSUMING the plastic is strong enough and your method of fastening is adequate, we'd still need to know how far apart the 2x4s are spaced and what area you're in.
That being said.....no, I do not believe it will be strong enough.
If you're looking at building this just as you would build, say, a shed but using plastic sheeting instead of roof decking and roofing material, you need to keep in mind that the decking provides lateral support and stiffness to the structure.
If this were my backyard project, I'd build this just as I would a shed or garage, with the same size framing, then use longitudinal 1x material across the rafters of the shed/gabel/whatever roof you're looking at building (as well as the sides assuming they're going to be unsheathed).
Then I'd take a good hard look at the strength of the poly and how to properly fasten it.....maybe someone with more knowledge than I have will come along to give you some info on poly.