I replaced my full stock on my 2006 ICS M16a3 with a Classic Army crane stock a few weeks ago. I guess I should have took some pics during the install.
It wasnt very difficult and turned out really nice. The only mod I had to do was a little dremeling on the ICS receiver stem splines to make them slide into the CA buffer tube and remove the wiring plug that was near the receiver. I then had to go down to the hardware store and pick up a 10-32 1" screw. This screw goes throgh the buffer tube and into the receiver stem which, on mine, was already threaded and where the ICS full stock bolted to. Therefore I didnt have to run the screw all the way into the receiver. The M4 might be a little different though, especially if it is not a full stock. There will also be some rewiring on a non full stock model to get the wires to the rear.
I have a 9.6v 4200mah crane battery using deans connectors in mine and the CA stock will retract fully.
Here is a pic of mine with the stock installed. If you need more pics or explanation when you begin the install, just let me know. I will be happy to take mine apart and take some more pics.