Have you checked out Hilleberg tents? They make 1-person, 4-season tents in the 3 3/4 to 5 lb. range. They aren't cheap, but they are very durable and are built by Swedes for extreme (Arctic and mountaineering) conditions. Take a look before you decide on a bivy that will weigh almost as much and leave you cramped, especially if you have to wait out a storm.