Hi King 767, Buzz here. Well...... it's sure an evolution of the JT9 series engine. Other that better materials, better oil system, better internal airflow, revised position of the accessories.... it's pretty much the same.(grin)
OK, to be serious, it's taken the good points of the JT9's and improved them.
I'm not well versed in the better materials depratment, but the oil system is made to keep oil in (compared to the old drippy JT9's) so they don't drink much oil per hour. Better airflow, more compression per fan stage
The accessories like hydraulic pump, fuel/oil pump, fuel control, oil tank, starter , etc is mounted on the bottom of the fan case ..... which makes it more convienient for guys like me who take care of them.
I guess the PW 4000 series has the ability to grow into a really high thrust engine, which the JT9's had kind of reached the end.
g'day and Good Christmas!