As phosphor112 said we don't know and the Technoblog pictures, which I think are fake, would point to little difference except 1.1 pounds lighter and a cheaper blu-ray drive, that's the popular opinion.
There are multiple points of information that support features not in the current PS3s.
1)
Power modes and EPA Goldstar rating
2) Kinect like interface for a depth camera (Digitimes PS4 rumor and Sony publishing a depth camera patent)
3) HDMI pass-thru (leaked Xbox 720 powerpoint Xbox 361 for ATSC 2.0 XTV so Sony might do the same. NASNE can support ATSC 2.0 and cable XTV better so this is not locked)
# 1 above is the big one as it means the 16 gig flash drive is in all PS3 4000 chassis and used for the OS and swapping the HTML5 stack in and out of memory. The PS4 appears to have a 16 gig SSD flash also for the same reason (allows the Hard Disk to sleep reducing power at the Menu screen).
Read the posts on the page above you!
Sony knowing (remember Sony and Microsoft were part of the group that helped set the ATSC 2.0 standards) that ATSC is going to be authorized/implemented late 2012 through first quarter 2013 would impact the design of the PS3 4000 chassis being released at the end of 2012.
Sony Media Fusion proposal http://www.atsc.org/cms/pdf/pt2/Naohisa-Kitazato-Next-Generation-Broadcast.pdf
Broadcasters Perspective on ATSC 2.0 http://www.ieee-bmsb2012.org/images/program/BMSB2012_Keynote_3_Connected_TV_Chernock.pdf
What should it have?
1) HDMI pass-thru
2) Complete redesign to comply with EPA Gold Star third tier Power modes also voluntarily required July 2012
3) a modern browser (Confirmed)
4) Skype or something like Skype
5) Voice recognition
6) Depth camera for more accurate gesture recognition.
7) Software to support an OTA digital tuner or accessories that do this connected to the home network (Confirmed Nasne)
8) Software similar to Google TV with ATSC 2.0 XTV extensions
9) DLNA/RVU/DTCP-IP software support (Confirmed)