Just seven months after cracking the $1000 barrier, Vizio’s 4K TV sets are getting even cheaper. Vizio’s new M-Series 4K (or Ultra HD) televisions start at $600 for a 43-inch model, and scale up to $4000 for an 80-inch model. Spending $1000—the same price that Vizio charged for a 50-inch 4K TV last fall—will now get you a 55-inch screen …