how do you expect anyone to answer that when they don't know how many cubic feet the place is?
4 * 8 * ????
I will assume here you are talking height and width, and assume that the tent is no more than say 3 or 4 feet deep?
4*8*4 = 128 Cubic feet.
You need to exchange that 3 times per minute so you will need a minimum of a 360 CFM fan and filter set-up.
Both sizes will work for you, the only difference might be the added drag in the 4" ducting due to the lower debit able to go through it at a time, hence a higher velocity of air movement.
a 6" will ensure less drag on the Fan and be generally a bit quieter if stealth is an issue.
hope this helps.