I actually came to the boards to see if there was a post on this already. I've won 6 games and lost 1 today, but I like to play a lot of different roles. I had already been placed in mid and jungle and wanted to play some support. After all was said and done I didn't rank up in anything but support and got less LP in mid and jungle than I would have by simply winning one of those games on either of those roles. (+18LP total - a win would have garnered me 22-28 LP). Needless to say that's kind of underwhelming. I think this system is fine for those who only play two roles all the time, but a lot of players like to switch things up; I think because the game gets boring otherwise.
Anyways good on you for posting this. Hopefully we can maybe get a bigger 'splash' LP if the roles are close in rank. Maybe 1-3 divisions you maybe get 10LP+ for other roles close to the rank of the role you're playing). +3 LP or +4 LP for other roles seems extremely low - especially because as the OP has said the role doesn't matter all the much especially if you are used to filling as your main.