Is this shelter for living in or surviving in?
If its for living, I don't know of anything that compact that is 4-season.
If its for surviving, then I might suggest two large orange trash bags and a mylar emergency blanket. One bag for the top (with a hole for your face), one for the bottom, and sit on the mylar blanket w/ leaves & debris for insulation.
Unfortunately, we can't pick and choose what time of the year a disaster might happen. If you had to bug-out in the middle of a freezing winter, do you think some garbage bags and a mylar blanket would be good enough? Personally, I would like something more than that.
It's true that I can't fit the best tent in the world in a backpack, but some kind of tent is infinitely better than no tent at all, IMHO.