See here for a step by step guide on how to retrofit your NG4 unit. It's in French but you can always use google translate.
http://peugeot-rcz.forumactif.com/t1652-procedure-d-installation-rd4-vers-rt6-ng4-wip-com-3d
I have retro-fitted the newer RT6 Sat Nav unit to my car, so there's a few pointers I can give you.
The wiring for the standard stereo is not compatible with your new NG4 sat nav, so you either need to modify the existing wiring or buy an adaptor cable. You can buy an adaptor cable here -
http://www.phibu-electronics.com/interface-evolution-rd4-vers-ng4-c2x15535502
I bought the adaptor cable, so I'm afraid I can't help you if you want to attempt to modify the existing wiring.
You will also need a GPS antenna and a longer (non-standard) USB cable to go between the headunit and the USB port in your armrest. The following USB cable should work, although I haven't tried it -
http://www.phibu-electronics.com/faisceau-usb-pour-rt6-rd45-c2x15252439 I made my own cable which probably worked out cheaper, this is fiddly but I can give you the details if you want.
One little thing to be aware of is that you will you lose the ability to change car settings (eg. switch on/off drls) once you fit your NG4 as it doesn't support this. Factory fitted NG4s come with extra buttons by the handbrake and the larger instrument cluster display to provide access to these settings. Incidentally, the newer RT6 can support changing these settings so doesn't have this issue.
As others have mentioned the unit will be locked to the VIN number of whatever car it came out of. You will need to use something like a Peugeot PP2000 box (Google it) to reprogram the unit to your VIN number.
Good luck and let me know if you have any more questions.