I'm not quite certain, but I believe the biggest difference is that the 4x4 JERRV has the Gyro-Cam.
I know the USMC uses the 4x4 because I photographed it at Camp Legeune.
At any rate, my walk around shows both of the above. I thought you had purchased it.
As for converting the 6x6 to a JERVV, I have all of the measurements necessary to do that, (Gyro Cam housing included) plus interior shots of the JERRV version, the locker, and where everything is stowed.

gentlemen good morning,

first let me provide my qualifications before answering some questions regarding the JERRV and the 4x4 Cougar, that way you can make your own assessment as to the accuracy of the information provided. I was the commander of an EOD Company in Afghanistan from 08-09 with 7 Army EOD Teams and 8 AF teams and then the CJTF Paladin J4 from 11-12 responsible for the distribution, maintenance, and upgrades of all Paladin JERRVs and Cougars.

Now to the heart of the question, the JERRV was only configured in the 6x6 version. There are no external differences between a JERRV and a 6x6 Cougar only internal differences and those are that the Cougar had the troops seats while the JERRV had the robot and ammunition lockers placed inside.

Regarding the 4x4 Cougar, the Navy, Marine Corps, and the Air Force utilized the 4x4 Cougar, the US Army did not except through loans from the US Navy. So there were US Army EOD Teams running in 4x4 Cougars but they were provided by the Marines or the Navy. The Air Force was not keen on loaning out their trucks.

With regards to the Gyrocam, in 08-09 there were limited numbers in theater so not all EOD Teams utilized them and some even had them removed from their trucks. We had more gyrocams during the 11-12 time frame and it was more prevelant for all service EOD teams and on both the 4x4 Cougar, 6x6 Cougar and the JERRV.

The internal configuration of the JERRV was pretty standard with regards to the lockers and the shelves however the loading of equipment and ammunition was team dependant. I had some teams that stored their stuff inside and some that stored it outside.

Andre my recommendation is for you to identify what time period you want to replicate as there are some other modifications that were done to the JERRV in the 2012-2013 time period and also what service you want to represent.

I hope this helps and if you have anymore questions please let me know. I will dig through my old photos and see if I can come up with my photos of the JERRVs and Cougars.

Dave