(click to show/hide)Why at the beginning Jigsaw is at the morgue and at the end he is where the part 3 end
(click to show/hide)Part 4 takes place at the same time as part 3. So in the beginning we see him in the morgue, but then it jumps back in time and shows us a story parallel to the one in part 3. Obviously they didn't make the time jump very clear, as that would spoil the entire experience, since this is the "twist".