$580 retail in OZ, similar price to a silver.....but doesn't do near as much.....half the size and 40% of the weight....can double as a discreet street shooter( but the Sony QX series are better imo)
no housing so battery is sealed in and supposed to go 2 hours
the sub 100 buck Asian copies of GP do as good a job, but same size as a regular GP
This website did a good comparison: http://www.gizmag.com/gopro-hero4-vs-gopro-hero3plus/34090/
Main points seem to be 120fps in 1080p (for the GoPro4 Black), a higher bitrate (60Mb/s), A/D audio converter, a 4K 'Superview', video and stills for 'Protune', and Bluetooth added for wireless connectivity.
Not sure whether many punters would benefit from the upgrades - the main one seems to be the frame rate which will be good for True HD slo-mo clips.
the GP 4 Session is a new model.....sitting in the middle of the range.....the specs don't compare to that link