The easiest way to move a heavy box up 4 feet would generally be to slide it up a 8-foot ramp.
Using a longer ramp requires less force to move the box upward, as the angle of elevation is lower. This reduces the effort needed compared to lifting it directly or using shorter ramps, which would require either more force or could be steeper.
So, in summary, the 8-foot ramp would be the easiest option among the choices provided.