I don't know what will solve your problem, but I know what the problem is. Pay attention to the 3 older girls, when two are playing together, do they argue as badly? Is there more arguing when all 3 are involved? I have never seen 3 children get along without squabbles. Whether it is boys, girls, or a mixture. Odd numbers of children, seems to force one to feel left out or something. Three seems to make for more competition or conflict or something. When the baby gets old enough to play with the 3 year old, things should smooth out some. Having all girls may always prove to be trying.