We had 5 people on our last trip. I just had my sister check in 2 of us and I checked in the other 3 ( we all had different confirmation numbers). I checked in my oldest daughter first & she ended up at the end of A. My DS11 got the highest B out of our group. Then my sister, my DS7, and myself were near the back of B. When my DD and DS got on they just went near the back of the plane. One of them spread their stuff out onto 3 seats and one spread out onto 2 seats. When we got back there, there were still lots of empty seats so it's not like they were keeping people from getting seats.
I think you and your 2 youngest kids can get on with the child boarding. They have no way of knowing you have other adults with you so its not like they are not going to let your other child on with you. As long as your DH and other kids are in B, they should have no trouble getting seats together, especially if you sit near the back of the plane. I wouldn't pay for EB, it really adds up when you have a large party.