Another thing about the 4D would be as SuperDave pointed out to me is the ability to have an internal drip pan without having an impact on temps! worked great for me on a 2 brisket smoke and clean up was a snap, and able to reuse bottom foil and woodbox foil over several smokes. Dave could chime in with more on this. Don't know if you could do the same with the 3.5 or not. I have never regretted getting the most real estate you can afford, not once.