You usually don't need a fan on the intake, so don't bother.

If you feel you NEED an additional fan on the intake (working together with the exhaust fan), when exhausting through a carbon filter (to remove odor) you want the intake fan to be SMALLER (lower cubic feet per minute, CFM) than the exhaust fan. That way all the air leaving the grow chamber will pass thru the filter. Otherwise, you may leak some odor through cracks.

You never want to use only an intake fan for the same reason: the positive air pressure in the chamber may cause the smell to leak out without passing thru the filter. Ideally, your chamber will be NEGATIVELY pressurized, so that if there are any little cracks, clean air from outside the chamber is leaking in (rather than smelly air leaking out).

Also, remember that the air is moving through the 4-inch exhaust fast, and that you may want your passive intake hole to be a little bigger than 4-inches (since the exhaust fan has to work a bit harder to suck the air in through a 4-inch intake than if you had, say a 5- or 6-inch intake).