This is the basic introduction of the Zeitgeist Adventure Path campaign setting. I wouldn't call it steampunk, but it adds renaissance politics and steam engine technology to the standard D&D fantasy setting.

I would very much recommend the Zeitgeist campaign, because it is full of interesting moral decisions for the players. There is no simple good vs. evil here, but various factions and philosophies in conflict with each other. If anybody ever told you that you can't roleplay in 4E, this is the proof that you can. The player's guide introduces special background themes for the player characters that help anchor them in the world and provide many story hooks throughout the campaign.