Once the ports open the power stroke is over! 4 strokes get a whole stroke for intake and and whole stroke for exhust - yes there is some overlap but not nearly as much as a two stroke...
The poppet valves of a four stroke also open well before the piston reach's top and bottom center. The exhaust valve opens before the piston reach's bottom dead center, though maybe not as much as a two stroke, At top dead center the amount the intake valve opens is not that much differant than the ports of the two stroke.