[Mike] is an avid PIC developer and replaced his ICD3 debugger for an ICD4. He made a video with his impressions and you can see it below. [Mike] found the heavy aluminum case with a sexy LED attractive, but wondered why he was paying for that in a development tool. He was also unhappy that they replaced the ICD3 cable connections with new connectors. Finally, he wished for the pin out to be printed on the case.
On the other hand, the ICD4 will also do JTAG and handle the Atmel parts (which Microchip acquired). [Mike] opens the box and shows the inside of the device before actually using it for the intended task.
As you would expect, the ICD4 integrates well with MPLAB and does the same kind of functions you expected from the ICD3 and other Microchip programmer/debuggers. [Mike] found the programming algorithm was a little different from previous devices, so he put the scope on it and also compares the speed of programming between a PICKit3, an ICD3, and the ICD4.
ICD4 is a pretty serious tool. If you want to just experiment, you can build your own PIC programmers or try the PICKit 3. You can also pick up a cheap board and go the online IDE route.