well without respect to anything, if 4.0L makes 250bhp, then 4.6L will make 300-312bhp. displacement changes change peak power in a linear fashion.
what that doesn't tell you is that while the powerband will increase linearly across the board with the increase in bore. the powerband shape, and time to accellerate will change with the change in stroke.
which starts a whole other can of worms also. if you're staying N/A, there's no reason why you can't bump the compression several full points & get more aggressive cams to offset each other. porting & polishing w a 3 angle valve job & unshrouding the valves goes along way also.