From a distance CombineZ4 and 5 look similar. Indeed, most of the keys work the same, the viewing modes are the same, and the layout is the same. The first major difference you'll notice is the absence of a Special menu. Its functions have been scattered amongst various other menus. The Filter, Undo and Macro menus are new.
Several features have been dropped, most notably Batch Processing and the ability to delete Rectangles from frames.
Macros are an exciting new feature. Initially they replace the Do Functions from the old Special menu, but because they are editable you can alter the parameters and functions used to do these tasks. A criticism of CombineZ4 was that you could not easily alter parameters. Now you can, and more.
Filters are also new to this version. A Highpass filter can be used to replace the old Sharpen function. You can now Lowpass filter the Depthmap to smooth out irregularities.
Automatic Size and Alignment is now more accurate. Find Detail is a replacement for Edge. Fill Gaps is a partial replacement for Despeckle; it gives a much smoother result over larger gaps by using a technique that I have named Depthflow. Remove Islands performs the other task that Despeckle used to do, i.e. removing small patches. Remove Colour Steps often helps remove persistent ghost edges where contrasting colours meet.
The Depthmap is now calculated to a greater accuracy, giving a smoother look to it. To go hand in hand with this I have added Interpolated Output; if a point lies halfway between two frames it becomes the average of both.
Several other items have been dropped from the menus and a few new ones added.