What's whining trans or rearend?

I just replaced the pinion and carrier bearings in my truck because of a whining sound but it whined all the time even in neutral. I finally put it up on jack stands and chucked the front tires and spun it up then I could tell where it was coming from. Yours being in 4th gear that might not be a option but I would think if its in the rear you would be able to hear it in first if your outside the car.