The difficulty with (4) is that it defines one constraint for each subset of a given set. This cannot be stated explicitly in AMPL, but a way to express it in AMPL is given by the following FAQ entry:
How can I get AMPL to index over the “power set” consisting of all subsets of a set?
Also, it is not clear how V and n are supposed to be related in (VRP1). I would guess that V is the same as 1,\ldots,n, and V-\{1\} is the same as 2,\ldots,n.