Overall the new processor makes a huge difference, everything works smoothly with little to no lag, and AirPlay is free of all the previous streaming bugs.
I was wondering about AirPlay. Is it a lot better? It has never worked 100% reliably for me with the ATV 3. Slow load time for 1080p videos recorded on my phone, occasional failures, and often I need to reboot the box. Would love to hear more about whether you experienced the same with the old model and how much it's improved with the 4.