Define 4d again please
Indeed, we are in a 4D universe.Yes, in physics, 4D refers to the idea of time as a fourth dimension, added to the (3D) spatial dimensions.
This is THE wrong reason for a 4th satellite.For those who can't get their heads round Einsteinian four-dimensional spacetime, here's a simplification.
You need four intersecting spheres to exactly pinpoint your location, and hence you need FOUR satellites (see attachment).
I remember you, mchambin, saying that this was the basis of early GPS algorithms.