4dr and coupes have the exact same engine and hence same power output. 4drs being just slightly heavier obviously so power to weight ratio is jst a little lower than a coupe. The tiptronics are great but obviously depending on how much power you plan on running there are various ways to strengthen the box. Also 4drs come in manuals too, you just need to look a bit harder and pay a bit more
As for the thing in the back window, no idea what you are talking about..a pic might help.