let base be 'x' and altitude be 'y' then the area will be 'xy'

given;
base increased as, x + 0.3 x = 1.3 x
altitude decreased as, y - 0.2 y = 0.8 y
according to given condition, the area will be 1.3 x * 0.8 y = 1.04 xy

1.04 xy - xy = 0.04 xy (which 4%)

that's why area increased by 4%