Spacetime intervals are always relatable and very well defined. There’s never one right reference frame that is better than another in some way, such that when

Spacetime intervals are always relatable and very well defined. There’s never one right reference frame that is better than another in some way, such that when you interpret the spacetime interval as space and time it is More Right than another interpretation, but all the interpretations are predictable. In this case it means that if the probe arrives at Mission Year 20 Week 0 according to a reliable onboard clock, and broadcasts it 4 light years back to Earth, then Earth will get this signal on Year 24 Week 5.