Why are the B-47’s inboard and outboard engines mounted so differently?
The forward pylon mounting of most jets with wing mounted engines is to use the engines' mass to move the overall center of mass of the wing forward, closer to or ahead of the torsional axis of the wing box.  Boeing discovered this helps with aerolasticity related issues with long, flexible, swept wings.  A bit like mass balancing a control surface, but on the root structure.  

The outermost engines, being so close to the tip where the chord is short, have to be farther aft or they would move the wing's center of mass too far forward in that area.