I'd say '06 or '07 is a little early. They've probably won't be done releasing all the extra's you need for SR4 by then (not that it's stopping them from still releasing SR3 timeline stuff now, mind you, but in theory...). They should milk SR4 for all they can -- magic book, cyberware, book, gun book, car book, matrix book, SotA book or two -- before they move on to SR5. Id give it maybe '08. Then not a new one, if the game is still going, 'till '12 or '13, easy.

I don't predict a "D&D 3.5," in other words.

All the supplements for SR4 will be compatable with the revamped SR5, with maybe some minor tweeks. Kind of like how the SSC and RBB worked for SR2.