I'm currently playing JC3 and it feels really shallow. All of the settlements and bases are nearly identical. Either fuel tanks, satellite dishes or electric boxes to shoot at and about 4-5 different types of foot soldiers.
With FC4, it's still a lot of doing the same thing, but at least you have hunting, crafting and other exploration type activities. JC3 has things to find, but the skills are tied to completing the same type of challenges over and over. I also found the controls in FC4 to be much tighter while JC3 sometimes just doesn't work.