If we assume teleportation or instantaneous travel were possible, and we could instantaneously travel to a planet 4 light years away and then return to Earth, we would essentially experience a time difference of 8 years.
Let's break down the scenario:
Initial teleportation: You teleport from Earth to the planet 4 light years away. Since the distance is covered instantaneously, no time would elapse during this teleportation.
Time spent on the planet: Let's say you spend some time on the planet, for example, a few weeks. During this period, time passes normally according to the planet's local time. So, if we assume their measurement of time is similar to Earth, a few weeks would pass on the planet.
Return teleportation: After spending time on the planet, you teleport back to Earth. Again, the teleportation itself would be instantaneous.
Upon arriving back on Earth, you would find that the time difference between when you left Earth and when you returned would be the sum of the time spent on the planet (a few weeks) and the time it took for the initial journey (instantaneous). In this case, the total time difference would be 8 years, as you mentioned.
From Earth's perspective, it would seem as though you traveled 8 years into the future, rather than into the past. This is because the time on Earth would have progressed while you were away, and you would be experiencing the effects of that time passage.
However, it's important to reiterate that teleportation or instantaneous travel is purely speculative and not currently supported by our scientific understanding or technological capabilities.