Often times in problems like this, it can be helpful to list the all of the possible ways each outcome can happen and then determine the probability that each outcome occurs from there. Below (sorry it looks bad - it won't let me paste a picture), I've listed all of the ways that a family can end up with 0,1,2,3 or 4 boys. Then, you divide the total number of possibilities by the # of possibilities for a outcome to get the probability that an outcome occurs. Last, multiply the probability by 100 to get the expected # of families in each outcome.
0 GGGG 0.0625 6.25
1 BGGG GBGG GGBG GGGB 0.25 25
2 BBGG GGBB BGBG BGGB GBBG GBGB 0.375 37.5
3 GBBB BGBB BBGB BBBG 0.25 25
4 BBBB 0.0625 6.25