if the magnitude of the four velocity vector is always c then it can be rewritten in polar form with 3 numbers giving the direction that it points and a fourth number giving the magnitude. since the fourth number is constant it adds nothing to our understanding so it seems like it should be dropped. also what is four force? does force have a component in the time direction? does an electric field have a component in the time direction? I just accepted these things before but I'm seeing now that I dont really understand them at all.