Not sure if you're asking about the 6" desktop fan or the 4" inline fan, but you don't need either fan to push fresh air into the tent.
The 4" inline fan setup to exhaust air out of the tent is sufficient to passively pull fresh air into the tent, that is your negative pressure.
However, you need to allow for air to be passively pulled into the tent without the flaps open during flower while the lights are off, unless the room is 100% dark the flaps will let light into the tent. I use 4" and 6" ducting in the unused port holes in the tent, make them long enough and the light cannot get up the duct into the tent.
I am using two inline fans, one 4" and one 6", because I have them. In the summer, I use the 6" to exhaust the tent out the window, and the 4" fan is an intake fan bringing fresh air from directly outside into the tent. I run the lights on over night during the summer to take advantage of lower night temps outside, I use that air to cool the tent. The AC Infinity controllers allow each fan to run independently and keep the perfect temperature.
The problem with inline fans is none of the inline fans on the market allow the user to set a constant low speed and then a much higher speed that only kicks on if the temp or humidity goes over a certain maximum. But even if they did work that way, I would still need two fans - one to recirculate air from the tent into the room, and the second fan to exhaust air from the tent out the window. One fan cannot do both, but one fan needs to be running 24/7 to maintain air exchange in the tent. To do this in the winter and not lose all your heat, that fan has to exhaust into the room. Therefore, a second fan to exhaust only excess heat out the window.
In the winter, I don't want fresh air directly from outside, it's too cold and there would be ridiculous condensation. The house I live in is a renovated 100 year old farmhouse so it's still pretty leaky, and I get plenty of fresh air drawn into the room from the rest of the house.
