Porsche is already working on the 4th model even if they say, they haven't make up their mind yet.

Last fiscal year (02/03), they have spent around 590m euros in investments (capex) mainly because of the Cayenne and this year they will spend around 500m euros which sounds a lot for the 997 (mainly) and a bit for the Boxster (987). They have to spend some cash to built the new engine factory which will have an annual capacity in excess of 100,000 units ..... there we are .... they need the 4th model to break the 100,000 units mark ...

So I believe within the 500m euros budgeted for this year, some is dedicated to the 4th model.
By the way, R&D budget will stay in line with last year's budget .... 500m euros i.e. a very high level for a firm of this size ... it is therefore quite clear the 4th model is on its way...