df %>%
group_by(col_a, col_b) %>%
summarise( a = sum(col_a == 'a'),
      b = sum(col_a == 'b'),
      c = b/a
)

df.groupby('id').agg({'out':[lambda x:sum(x==4)]})

df.assign(out=df.out==4).groupby('id').agg({'out':sum})