It will probably be a cpu refresh with newer 6th gen from intel. I hope they do not make the size bigger for SP4/S4 because the current screen size form factor is really what attracts me. To be able to reuse their accessories on newer gen will also be nice.
The SP4 should be released first and S4 later since MS released S3 just around March this year while the SP3 was released last year?
The big unknown is whether they choose to go fanless with 6th generation Core M, or keep the fan and go with 6th generation i5 and i7.
As for screen size, it is possible to take the same form factor, enlarge the screen size a bit by shrinking the bezel, and still fit into the existing docking station, which Microsoft has said will be compatible.
A good size would be 12.4 inches, which would give the SP4 the same screen length as the Ipad Pro. Microsoft can opt to go for 2700x1800 resolution, which would give the SP4 the same pixel density as the Ipad Pro.
A most ironic thing about the 3x2 aspect ratio of the SP3 (and presumably SP4) is that it is the exact aspect ratio of two Ipad 3x4 screens side by side!
Other possible useful improvements
* option for LTE
* higher maximum brightness for the screen
* windows button can either be removed or replaced by a fingerprint scanner
* an extra USB type C port
* further improvements to the type cover and pen
* auto focus for the cameras
All speculation here, of course!