Listed below are the SIXTEEN (16) different possible outcomes (in terms of gender and order of birth) for a four-child family, using B for boy and G for girl (and going from oldest to youngest.)
Note the outcomes of 2 boys, 2 girls (in red) – there are only 6 of them.  (So the chance of having 2B, 2G is 6/16).  Also note the cases of 3 of one sex, 1 of the other (in blue), of which there are eight (8).  So that likelihood is 8/16 or 1/2 or ’50-50′.

BBBB

BBBG

BBGB

BBGG

BGBB

BGBG

BGGB

BGGG

GBBB

GBBG

GBGB

GBGG

GGBB

GGBG

GGGB

GGGG