Historically speaking, his not attending his successor's inauguration should more properly be seen not as his being the 4th-ever sitting President to skip his successor's inauguration but as his being the 1st-ever sitting President to have not been invited.
At some point inviting Trump to the Biden Inauguration might have been considered akin to either a plot or story from a William Shakespeare historical, or maybe something out of Game of Thrones.
Don't blame "The Crow" if the story is true, actually makes me respect him more....