How much does it cost to dig trenh for water line from house 30ft long and 4ft deep

Depends on where you are in the world, where you are in a country, how rocky the soil is, and how much you want done to put all back after the line is done.
Call 2 contractors locally and pick one. They will know the terrain local to you.
Call Dig Safe or whatever it's called where you are too. They will mark all existing lines, wires, and cables.

A lot, or a whole lot depending on what you don't know. joypulv described some of those variables.

It really depends. Several factors will determine the actual cost of excavation. As suggested in above post - call several contractors for free estimate and pick the one you like.

Milo