Do 4 year olds know the difference between a boy body and girl body.
Not in a vital way, I would argue.
Hence their disagreement would be with the identity they associate with each sex—boys become president, girls play with dolls, etc.. I’d take their disinterest in whatever they associate with their given sex, therefore, to simply be reason for broadening their idea as to what boys and girls like and can do. Boys can like cooking and girls can like sports, or whatever.