Are there any bonus for playing 4 v 5 games?

In short, No there is no current compensation, or any plans of a system to do so. However, the system is not completly without forgiveness or mercy and as such Riot has implemented a 'Leave Buster' system, which punishes players for leaving matchmade games early. So no, you wont be compensated, but they will be punished for leaving.

how Ender said, no, probably because people will leave the game to give that bonus to the team and then they will connect. possibly what riot should do is not to let it reconnect after a while and give the bonus. so far the only thing you get in most cases is a bad game.

So no you won't get anything from winning a 4 v 5 in terms of LP or XP. Any ways I just finished a game were yorik was afk. The only reason he was afk is because he wanted to go top but Lucian picked it first so he rage quit at level 1. I was jungle shyvana and it was a very difficult game to pull one of the most rewarding wins i had in normal games. I think that the best thing 4 v 5 games leave you is the knowledge and the confidence that you can still win. That game was all about me peeling my two adc lol! so yeah don't be discourage.