Feels like people will be urged to move to Python 3 due to the new features... And yet I'm thinking that Python 4 will happen before many have been able to move to Python 3. :-P
Because Python 4 will be only "whatever comes after the last 3.x", with no backwards-incompatible changes, it won't matter: porting to "python 4" will require the same effort as porting to 3. So start now ;)