I'm not an expert in aircraft design. I could imagine, that the engine position is actually influencing flight stability and flight behavior of the plane.
Seeing the two-eng wing-mounted as a "normal" way, 4 on the wing still pretty normal, DC-10-style to increase vertical maneuverability (is this a word???) by a third centered thrust. I read in a book, that this layout helped a crew to land a DC-10 wihtout rudders, to control bank by the wing engines, and descend and levelling out by the tail engine.
I also read, that there is an effect, which uses the exhaust stream of the engine to flow over the wing, creating more uplift.
It probably also depends on engines on the market and their power. If, like Coronado mentioned, exhaust in inflow wouldn't be a problem - what sense would it make to mount 4 engines to a 737 - two on wing, two on tail - would probably be great looking, but probably no sense in economical ways.
See also the thread about why the BAE146 has 4 instead of 2 engines...
The DC-10 had pretty big engines, and placed in a triangle, which would, like mentioned enable the crew to use the the engines for steering. I don't think, a "4-engine, 2-wing 2-tail concept" would provide these effects as well.
Just my stuff, I really am not an expert, this is what I gathered by reading and thinking. Interesting thread though!
Putana da Seatbeltz!