i think fit wise, should be able to go in. the chassis is the same, and the fitment for the springs and shocks are all the same.
i can only imagine that the springs and shocks for the front are slighly different for the 6 cylinders to cater to the extra bit of weight. if you put the 6 cylinder into the 4 cylinder, the front may be a little higher than usual. but it shoudl be able to fit
actually i'm not even sure if they have diff part numbers for the 4 and 6.