most of the time when a game becomes 4v5 due to a leaver, the team with 4 players will lose. Riot should give Baron Nashor aura or upgrade stats to the team with the missing player to even out the game. or possibly give AI control over the missing player's character.

i think they should just add a bot

Though in ELO, players should be able to kill a bot...

Bot no.
Give permanent Baron stats to team with 4 people UNTIL the player comes back, yes.

Just won a 4v5 game, but he left right at the start, that makes it possible.

It's better to hope they will reconnect in time, sure sometimes people have bad luck, but if there would be a system to help the remaining players there is less reason to stay, people might leave for a small reason where they would otherwise stay.