Lucid no one knows for sure, but the current Audi way is to offer only one RS at a time. Simple deduction (RS6 imminent, RS5 heavily rumored) puts the next RS4 at about 5 years out. No way to be sure of that, but I would personally bet money its no closer than 3 years. And that would be if they decided to do the RS5 coupe/cab and RS4 sedan/avant all at once, which is a valid possibility given they are basically brothers just like E90/E91/E92/E93. But either way, it will definitely all have to wait until after the new RS6 runs its course.
Oh, and also there is the wild card RS R8 that might fall in there someplace too, and push the RS4/RS5 out further as well.
2008 Interlagos Blue E93 M3 - M-DCT
2011 Salsa Red Jetta TDI Sportwagen - DSG