Fraction of a whole: (fraction) x (total) gives the part. Subtract to find the rest, then compare.

Number of boys	=	\(\dfrac{4}{9} \times 36 = 36 \div 9 \times 4 = 4 \times 4 = 16\)
Number of girls	=	36 - 16 = 20
Girls - Boys	=	20 - 16 = 4