2X4x5' tent ventilation

Other only 40cf of space what online fans can I use. Everything I see is overkill.

Questions:
Could I get away with just an exhaust fan and allow the vacuum effect to pull enough air into the room?

How would I contain the c02 during the flowing stage with such a high air exchange in a small spaces?

When I used to use a 2x4 I had a 4" ruck, its not a problem having more air exchange than needed. Plus your better buying a stronger quality fan and using a variable transformer to turn the speed down than buying a cheap ass fan that doesn't move much air to start with. The first is much quieter than the second.

Yes you can just extract air there is no need to use an intake fan, this is called passive intake. Active intake is needed in much bigger set ups.

You wont be able to contain co2 with an active air exchange. Half of the reason we exchange air is to maintain co2 levels.
If you want to contain co2, as in use a tank and add co2, then you need a sealed room. There is really no need to do that though.

I agree with @coreywebster. Just make sure you can dim it low enough. Too much airflow will lead to too low humidity, the kind of low humidity that won't be fixed by a little humidifier as the fan will just suck the humidity out of the air quicker than the humidifier can supply it. This can be remedied by setting the fan on a timer, but you'd be best to find a fan that can run constant. The temp and RH will stay more stable if the fan is running 24/7 from my experience.

I have 2 442cfm fans (I only use 1 in the winter), and it was kind of a bitch tuning it to my grow room. But I also have this fan cooling a 1000watt hid, the fan constantly kicks on and off to allow some heat to accumulate in the grow room so that I don't have to run the space heater as hard. With no airflow it gets way to hot, with too much I can hold my hand on the cool tube indefinitely and it's lukewarm. I found a happy medium, and even tried to mimic mother nature buy allowing the light to get hotter during the peak of the light cycle and stay cooler towards the beginning and end.