would this work?

i plan on growing 3 sativa plants in my bedroom closet. i would have 2 or 3 fans blowing through carbon pads for smell, and during the veg cycle and the light hours of the flowering stage, i would keep the door open for air exchange. do you think this would work? my lighting will be 400w mh and 400w hps.

why the fans blowing thru carbon filters? with the door open it cancels any stealth you may be getting using fans. fans on the plants, yes, definitely, butotherwise, not with an open door, unless you find a way to vent it with the door shut.

i don't want to invest too much in an air filtration/venting system. i'm only growing 3-4 plants, so will i need to worry about smell? will it go beyond my bedroom?

That much light is gonna create a whole lot of heat for a closet. I'd be worried that your ventilation system won't be good enough. Plus, be careful with the Sativas; they like to grow tall and lanky, so yea...closet with HID could be an issue.