Answer:  To place three trees equidistant (never minding the fourth for a moment), there must be an equilateral triangle -- the only shape that positions three points equidistant from one another.  The fourth tree, to be equidistant from the others, also has to make an equilateral triangle with each pair of them, so it has to be off the plane -- on a hill or in a pit.  As to the pit, drainage would be something of a problem, and the landscape architect certainly does not want a golden rain tree in a hole in any case.  To be practical, the forth tree would go on a hill.