While I know this is an older post. There is a little more to the differences in the 3rd gen and 4th gen Intels.
Not all but most 3rd gen intels are or can be Tray mounted or tray sockets or soldered on a motherboard
No 4th gens are tray mounted. They are soldered on a motherboard
3 gens run at different clock speeds the highest at 45 degrees where 4th gens run at 47 degrees
4th gens have to run DDR3L which is really nothing more than DDR3 @ 1.35 volts. But struggle or will not even run the DDR3 @ 1.5 volts
3rd gens can handle up to 16gbs of ram compared to the 32 gbs of ram that a 4th gen can run.
And there are a few other differences. But I think might be enough to help clear the picture.