While there are vehicules without 4 wheels, the 4 wheel vehicule is quite normal and natural and pre-dates the motor-car.
If you talk about coaches - yes. But they're an invention on their own, being pre-dated by one-axis/two-wheel vehicles (usually powered by 1 or more animals like oxes or horses).
Stability and space. 4 wheels, one of each corner is the most stable configuration that provides the most load bearing and interior space.
The most stable configuration generally is sitting on one or 3 points, not four.
A more or less square box with four wheels (one on each corner) is simply the most practical (read: economic) approach for mass-production, driven by simple technical requirements (e.g. common axis for the most economic connection of the axle driving shafts).
And then there is the habit - people are not allowed to _think_ about leaving the path where companies have lots of experience already. A big company usually is afraid of experiments (especially in the car sector).
One could probably win foot space if there would be only one front wheel running in the middle; there would be more room to enter the car without the wheels being in the way (especially for rear seat passengers) and probably some more aspects, that simply do not get investigated because car companies are traditionally ultra-conservative.
I could imagine that with the advent of decentral propulsion (e.g. electro motors flanged to each wheel individually, thus no need anymore to have the wheels aligned on a common axis for the most effective way of connecting the axle driving shafts) we might eventually start to see completely new concepts.
Same goes for a tablet: a flat rectangle is the most practical (economic) approach for mass production - but you would be free to make it e.g. a flat square or add some bells and whistles to it (like the bezel sitting on top of the glass plate, sporting additional buttons etc. - which is how early attempts of Tablet PC's actually looked like). The problem would be only that you'd need dedicated parts produced only (mostly) for you, which would increase your component price drastically compared to a hugely scaled mass production of "standard" rectangular screens with little to no external buttons, having everything inside the OS/on the screen!
Before Apple introduced the iPad, no-one went that way, simply because there was no OS available that would have allowed to do without quite some external buttons. Apple was brave enough to trail the path and actually patented its way of having a dedicated OS developed exclusively to drive touch-screen-based devices (it doesn't matter that they used a spin-off of OSX - iOS has lots of man-years R&D in it).
After Apple's success other companies tried to jump onto the bandwagon instead of sticking to their alternative paths (TabletPC, SlatePC, UMPC etc. etc.) or developing completely new approaches (e.g. have a metallic bezel sitting on top of the touch glass, have a coloured bezel (e.g. dark red for Samsung as found on their TV's), have a color-changing bezel reacting to machine conditions, have a screen-protecting flap, have touch buttons all around the bezel, have a separate entry touch screen in - say - the lower quarter of the screen, have the tablet back coloured and / or made of plastic, and so on and so on). They would have had lots of possibilities even within the rectangular form factor - but they chose to simply imitate the most popular device on the market, trying to participate from its "Halo" effect.
If four wheels on a car (including their positioning) didnt make sense, we would see companies pushing other (better) designs. Are we? No.
1. It was an (intentionally absurd) example.
2. I was talking about vehicles, not cars.
3. There are lots of vehicles with more or less than 4 wheels
4. The positioning also isn't the same on each of them, not even on the 4-wheelers (think of buses or small trucks, where the body reaches some 1-2 meter or more beyond the rear wheels...).
5. Companies are indeed looking into three-wheelers (e.g. the Mercedes F-300 Life-Jet from 1997 as one well-known representative of the Tilting Three-Wheelers category) - such cars promise less fuel consumption, more dynamic street performance, increased curve stability, less weight etc. etc.
