I don't know about behind the dash, but I recently did some work to replace one of my 12V sockets. See this post for directions on how to gain access if you have a 2014 and later LR4:
http://www.landroverworld.org/threads/voltmeter-usb-charge-port-install.29746/.
Pfunk951 further added photos and instructions for 2013 and earlier (traditional gear shift lever), so you should be covered no matter which year LR4 you have.
Additionally, if you want to tap in to power near the top of the windshield, you can follow umbertob's detailed directions from his May 17, 2015 post about installing a dashcam and using the power supply located in the overhead console. I did this about a year ago and it has been working great. I have copy/pasted his text here:
"I used the same blueprint that worked on my 2006 Sport. Although there is some additional stuff above the overhead console of an LR4 (the TPMS module, for one - didn't have that on my 2006), the basic wiring is exactly the same. To connect a radar detector and/or a dash cam that turn on when you unlock the doors and turn off when you lock them - or about 5-10 minutes later if you don't - you need a steady, 12V switched source of power up there, and a ground/earth point.
Always disconnect the negative lead from the car battery before you start messing around with anything electrical. Open up the overhead console: pull it down from the front edge with your fingertips and it will swing open like a book. Observe the two main harnesses connected to the dangling console: there is a striped green / white wire in the harness for the overhead map lights that works as a switched 12V lead, just splice your positive (usually red) wire with inline fuse from the radar detector or dash cam's hardwiring kit into that lead. I use 18-14 ga. rapid splices available at most hardware and auto parts stores, although many hardwiring kits include them in the box already. Careful as there is an identical looking, green/white striped wire going to the other harness for the sunroof switch and microphone. Don't splice into that wire by mistake, as it will not work.
Then, slightly loosen one of the many unpainted bolts up there and connect your negative/black lead from the hardwiring kit (with a U or O-ring terminal to secure it between bolt and washer, or just wrap the copper lead several times around such bolt) as your ground, and tighten the bolt. That's about it. Connect your device / camera / detector to the hardwiring kit you just hooked up, reconnect the battery and test. If everything fires up (and turns off when you lock the doors with the remote), as finishing touch use some small zip ties to secure and tie together any loose wiring and dangling connectors, and push the front edge of the overhead console up against the roof until it snaps back in place. Done."
