Not sure I understand why you would want to vent the upper bag outside, first it is a filter, so if you replace it with a 4 inches pipe, you will be sending the dust outside and keep some dust and heavier particles inside. It will not overload the motor is you put some restriction, 4 inch pipe is a restriction and surely sufficient, those fans can be used simply to circulate air in a 4 or 6 inch duct for air make-up units. Along with the dust, you will be sending a lot of heat also and you will need to make-up for the volume of air lost. If you do not allow air entry, you will starve the dust collector, it cannot pull more than the restriction of air incoming in your room. In Montreal, buildings are well insulated and well sealed and tonight it will be -19C, below zero as we used to say, you would be sending 570 ft3 of warm air each minute and replacing it with 570 ft3 of -19C air, this would work for a freezer but in a workshop, it will get cold and dry.