4" is the cut off for short arms. The problem being the control arms become too short and the work angle is too big. As you go taller, the axles are pulled inwards. On a stock Jeep, the control arms are almost horizontal. That's the way they were designed to work. As you lift the Jeep higher and higher, the control arms start pointing down. Ever hear of the TJ wheelie? This is caused by the torque from the engine twisting twisting the frame which loads up one side of the suspension and if you input a little additional lift turn to help throw the weight to the outside, the driver
front tire will pickup off the ground. Sometimes you can get the rear driver side in the air instead. Currie calls this 'Jacking'. The is also why some people convert their rear suspension over to leaf springs.
Honestly though, unless you want to go with more lift 5+ and 35 and up, there isn't a need to go with long arms. You don't rockcrawl and it doesn't sound like the trails are all rutted out so other then height, there doesn't seem to be much need for one.
