I haven't stayed at Poly during 4th of July, but I stayed there for the most recent New Years Eve on December 31, 2018 and can tell you what they did then.
They roped off the beach in the red circled area for guests of the Polynesian Resort and the Poly DVC Villas. Resort guests were issued yellow wristbands to indicate they were allowed entry into this area. The yellow wristbands were waiting for you in your room when you checked in. If for some reason you didn't get one/lost it/etc, there was a designated place to go to by the pool where you could confirm you were a resort guest and get issued the wristbands. It's possible within that red circled area there was another roped off area for those who might have been Club level or those who paid for some kind of extra experience, but I don't quite remember. But I can confirm there is crowd control for the red circled area of beach and it is designated for confirmed resort guests only. It was obviously a popular spot to be and a lot of people were there, but they were enjoying themselves. It was, in my opinion, definitely not to crowded to the point of it being unenjoyable or to ruin the party spirit of the night. There was room to lay down a blanket, or to walk around with your drink, and for the kids to play. It wasn't like guests were being packed in like sardines, shoulder-to-shoulder as in the parks.
I did not notice any kind of crowd control for other areas at Poly. If there were CMs going around checking magic bands to confirm you were a resort guest then it was subtle. But even outside of the designated red circled area it wasn't crazy crowded to where, as a resort guest, I felt like people who were not staying at the resort were somehow ruining my experience. I assume if they did have mass crazy crowds then CMs would have been going around and checking magic bands.
I noticed a number of people watching the fireworks from (both areas circled in black) the boat launch and top of the hill behind the bungalows. I didn't think to check west beach (circled in yellow) to see what was going on there. My favorite spot to watch the fireworks was the small portion of the beach closest to the Ticket and Transportation Center, circled in blue. I could hear the show music and narration being pumped in by speaker from all of these locations. If you don't like whatever spot you've selected for the first fireworks show, move around and pick a different spot for the second show.
My favorite thing about being at Poly for the NYE show (and they also do this in the 4th of July show) was the fact that you get the most amazing view of the dazzling display of perimeter fireworks and the fireworks being set off from one of the islands in the Seven Seas Lagoon. You wouldn't know it if you were watching the show inside Magic Kingdom, but there's a lot more fireworks happening during these shows besides what is being set off behind the castle. In my opinion, these kinds of shows are much better visually (and also better with respect to crowds!) from places like Poly or TTC than from inside the park. I think you'll have a great time at Poly for the 4th of July fireworks!