The final output shaft on the 4wd 420 runs from the rear of the engine to the front of the engine. You would have to completely disassemble the 2wd engine and split the crankcase and change the shaft from the 2wd shaft to the 4wd shaft and then switch the front crankcase cover. The first illustration below shows the 2wd final shaft and the second illustration shows the 4wd final shaft.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.