The downvotes are because you a making it seem like your "4hrs" of time is worthy of some god like recognition. In reality, 4 hours is basically nothing for an established project and you will be lucky to even begin to understand the project functions or where the best place to contribute is.

You are not going to hand out food at a shelter or rake someone's lawn. Participating in a project is not something to be taken too lightly. I would suggest you read up on some of the projects you like and what their goals or TODO items are. Then pick one and start contributing. Don't make it about getting karma, but about contributing.