In the summertime, and Fall/Autumn, and Spring, my normal, two-wheeled bicycle works perfectly well.
But then there's Winter. What is normally friendly, non-slippery asphalt and grainy forest paths suddenly have turned into slippery death traps. Any route I pick is now like a lethal obstacle course where I put my life on the line. One mistake and CRACK! There goes my skull. Or rib. Or any part of my body. While typing this, I have just come back from one such terrifying bicycle trip, and it was anything but enjoyable. Scared to death almost every single meter, I was forced to move extremely slowly, sometimes even walking with the bicycle by the side, constantly on guard and ready to engage "operation save my own life and body parts when the whole thing just falls to the side".
Even when they have "sanded" the roads, many places look basically like a pure-ice slope down to Icy Hell. I've had "near-death" encounters like this many times in the past, and I never forget how scary it feels, so I'm very much trying to avoid it happening again.
I can't help but think how great it would be if, instead of one wheel in the middle in the front and in the back, it had two wheels, to the left and right, both in the front and in the back. Spaced apart quite a bit. This, at least according to my understanding of physics, would almost make it impossible for it to turn over while I'm riding it, no matter how slippery and hostile the surface is.
It would also have the added benefit of making it much more doable to have somebody ride in the back in non-winter time, because the weight is distributed better and the two wheels provide enough support for another person, or higher load (such as lots of groceries).
I know that there are those "fatwheel" (I think it's called? Fatbike?) bicycles now, but that doesn't help in this scenario. It doesn't matter how thick the wheels are -- they have to be two ones that are placed apart, no?
Short of some amateur-built "crazy inventor" contraption, I've never seen a four-wheel bicycle in the wild or for sale anywhere. Why is this? Why not enable me to move around in the winter? Is there a good reason for why this is not done that I have not considered? And it's not like it would not work in the summer as well, so you wouldn't need two different bicycles. And if one wheel is punctured, maybe it could offer a way to continue on home with just the three other ones. I see only benefits, myself.