If you don’t listen super loud or sit too far away, and the speakers don’t present a difficult load, you may likely be fine without additional amplification. Again, this really gets into the minutia of an age old conversation.
I chose mono block amplification for my Front 3 and Surrounds because I am more into music, and I wanted the Surrounds included to keep the system matched for 5-channel audio.
That said, it’s nice to take some load off the AVR, and I would still recommend cooling for it as well, something like the AC Infinity Aircom T8 or similar (depending on your set up).
Regardless, I would suggest starting without. It’s easy to hear if you are getting the performance you want. D&M have a 70% guarantee on power delivery for all your channels. So take your stereo power rating and reduce that to 70% and that’s what you should be able to expect to be delivered to all channels when running your 7.x.4.
As far as the difference between the models… just shop for the features you want. If the 4700 does it all, go for that and plan on adding the amp to get the +2 channels. This is really a personal preference kinda thing at that point.