Of course there are lot of differences between DPP4 and DPP3 concerning the user interface, but I'd like to discuss here the differences concerning the proposed functions (not the user interface) by each software.
For the moment I have noticed the following differences :
1. DPP4 has a new white balance tool which integrates Magenta/Green cursor and Blue/Yellow cursor. DPP 3 has something similar but less convenient as there are no such cursors.
2. DPP4 has color based adjustments
3. DPP4 has no HDR or composite tools
4. DPP4 is slower than DPP3 when converting
5. DPP4 seems to have better higlights recovery and propose some extensions to the raw histogram
May be there are other differences I haven't noticed yet. If so please share them.
Here are some details about points 4 and 5.
Speed
I tried to convert the same file using both softwares and here are the times :
DPP4 : 30 seconds
DPP3: 13 seconds
Raw histogram
DPP3
DPP4
As you can see the range is larger in DPP4 in particular in the highlights. You can move the right limit at more than 4.0 in DPP4. Furthermore, you can also move the middle point in DPP4 which allows to better adjust the curve. And as an effect the range of the exposure slider in DPP4 goes from -3 to + 3 while it goes from -2 to +2 for DPP3.
It seems that this allows to better recovers the highlights in some cases as you can see in the following samples :
DPP3
DPP4




