In a recent post by Harmonix/Rock Band on Facebook, a Facebook user requested Led Zeppelin. Rock Band's page responded back saying, "RockBand.com/request. That's all we have to say. Thanks."
What could it mean? Nothing, possibly, but knowing Harmonix's past of generally not responding to requests for artists like Pink Floyd and Led Zeppelin, I would say it fairly strongly suggests that Led Zeppelin will be in Rock Band 4 -- either on-disc, or in the form of DLC.