It doesn't get any more lightweight than a good size nylon tarp and a plastic ground cloth. I spent many thunderstorms underneath one and I always carry a mosquito headnet for sleeping (one with hoops in it to keep some space between it and your head while you are sleeping) to keep the bugs away. A good tarp will keep you warm and dry.
I like a nylon tarp with some ties already sewn in. It allows you to rig it in a variety of ways. The ground cloth should be quite a bit smaller than the tarp so the rain doesn't come down the tarp and land on the groundsheet and soak your sleeping bag. If you need 4 walls, I'd probably just buy a good lightweight tent. I have a Northface tadpole that weighs in at around 4 lbs. Although it's listed as a 2 man, it gets a bit cramped with 2 people in there. Used it for several years then went back to the tarp although I occassionally take it car camping.
One of the downsides of the tarp is that it's not a standalone. The tadpole is (now called something else, I believe).
Bivvy bags are interesting but I feel really cramped in them. I rode out an incredible storm once backpacing next to Lake Superior while in a bivvy and didn't much care for it. I like a little room to spread things around. They work for some people though.
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Hoodoo
The low, hoarse purr of the whirling stonethe light-pressd blade,
Diffusing, dropping, sideways-darting, in tiny showers of gold,
Sparkles from the wheel.
Walt Whitman
[This message has been edited by Hoodoo (edited 05-11-2000).]