I would leave the 4WD off unless I was stuck and trying to get unstuck. If the weather is bad enough that you need to drive with the 4WD on, you should be driving slow anyway, so the question is kind of useless. It would be pretty foolish to drive fast in weather in which you felt it was necessary to use 4WD.
Oh, and no matter what you read here, your [i][u]owner’s manuals[/i][/u] have the definitive answers for both vehicles. Read them and let us know that they say.