Short answer: we're going with when it was written/vidded.
Long confusing answer: If it was written in season 4 but reaches back to season 2, then that is part of the "live what were we doing/feeling in season 4" experience. Someone in season 4 felt it was important to talk about season 2. and we felt it was important to read or watch the story/vid when it was written - in season 4.
If a season 2 story was published in season 4, then readers and viewers who are watching the show today in order will be able to follow what the story/vid is talking about (they've watched seasons 1-4 already). I sometimes have to remind myself that the events in season 3 or 4 may have changed how the writer/reader/viewer saw Sam and Dean or John or Bobby or Mary or Jessica or ... etc in season 1 and 2.
Time Travel - Real Time
Long confusing answer: If it was written in season 4 but reaches back to season 2, then that is part of the "live what were we doing/feeling in season 4" experience. Someone in season 4 felt it was important to talk about season 2. and we felt it was important to read or watch the story/vid when it was written - in season 4.
If a season 2 story was published in season 4, then readers and viewers who are watching the show today in order will be able to follow what the story/vid is talking about (they've watched seasons 1-4 already). I sometimes have to remind myself that the events in season 3 or 4 may have changed how the writer/reader/viewer saw Sam and Dean or John or Bobby or Mary or Jessica or ... etc in season 1 and 2.