Each of your tents is 4x4x6 = 96 cubic feet = 192 total. Normally it's recommended to exchange the air about once a minute with HID lighting, but a kw is quite a big light for that size tent, so it can't hurt to go a bit overkill with the ventilation to make sure it stays cool.
Plus the fact that a carbon filter, as well as any ducting, creates resistance and reduces the amount of air the fan can move below it's rating. To minimise this you should try to keep all ducting runs as short and straight as possible. If you have to put a bend in it try to make it as long and smooth as possible, rather than a sharp turn as that would slow airflow considerably.
Carbon filters are normally rated by the amount of air they can flow in cfms (
edit; a lot seem to be advertised rated on cubic metres per hour, but it's simple enough to convert, just type it into google
), so try to match that to your fan as closely as possible, but it's better to have a carbon filter rated slightly higher than the fan, as opposed to the other way round if you can't find an exact match (though if, say, you had a choice of a 300 or a 350, go for the 300 as it's closest to the actual rating of the fan)
I've never used a tent, or known anyone using one. I have a pretty low opinion of them myself, mainly based on looking at tents for sale and thinking 'I could build a fantastic cabinet for half that price' but have nothing against people using them as a quick setup solution, and obviously I see their merits, I just prefer to design and build things myself.
The advice I've given is relevant, since the principles of airflow remain the same no matter if you grow in a tent, cupboard, attic, basement or wherever. You take hot air out from the top, leave a hole for cool, fresh air to come in near the bottom and match the fan and filter to the size of the room, and the heat generated by the lights.
Anyroad, I think I've gone on long enough for one post, hit me back if you need any more info.