You need 4 signals to get 3 coordinates.

No, you don't. You need 3.

EDIT:

For the mechanically and geometrically challenged:

The signals are used to measure travel time, and, when multiplied with the speed of light in vacuum, distance between points. So, imagine that you know the distance from one definite point (the first emitter). That means that you are positioned on a sphere with the radius equal to the distance end the emitter at its center. This is a 2-dimensional continuum. Now, let’s say you also know the distance from a second definite point (the second emitter). This also determines another sphere. The intersection of the two spheres is generally a circle or a point in the extreme cases. Now, if you know the distance from a third definite point (the third emitter), then the set of the possible positions narrows down to the intersection of the third sphere and the circle, and that is a point. In other words, you determined your position uniquely. Put in another way. Try moving a body that is fixed with three rigid rods.