I really sympathise with you. I don't have four children, but I have two, and they've never really got on since they were early teens. They don't fight with each other, but they are just so different, that they aren't interested in being with each other.
It's really hard as a parent. Because no matter what I want, you can't make DC get on with each other.
And - as I have discovered - you can't engineer situations that will suddenly mean that they enjoy each other's company.
Mine will come out together with me and DH, and they are totally civil to each other, but it's painfully apparent that they are very different people and have very little to say to each other.
I can only hope that as they get older, family ties will become more important to them.
But it's really sad, and I hate it 