From the way the P3 updates were going, it seems likely the P4 will be subjected to ever more inbuilt restrictions - and increasigly hard to avoid being forced to apply.
If new to the market now, the 4 is clearly the one to go for though, provided you don't mind waiting till late spring for it to be widely available.
It will be a while after that before the 3rd party accessories start to appear.
But if you spent the last year or so getting together a good P3 kit with spare batteries etc there seems little point selling at a loss.
The P4 offers little by way of worthwhile new features to a more experienced user.