Simple serving 4 years and not running for reelection would not be the best move by Biden. Dems would lose the power of incumbency. The primary would be a free-for-all, and although it wouldn't be the joke of a mess that the Cons will have, Dems don't need that unnecessary instability.
I picture Biden resigning 2-3 years into his Presidency, allowing Kamala to run in her own right as the sitting President. She's not my first choice for #47, but this is the Dems best move. And yes, Biden will go down in history as a healing figure and symbol of hope during what will be a long recovery period.