careful on fan controllers the cheap ones ($50 - $150) will just make your fan louder by making it hum, yes it reduces the air flow which in turn quietens the out take but that hum from the fan may become louder than the out take air movement noise.... I've built silencer boxers for my fans (I do use larger than you) but in your situation that looks like its not possible... looking at that tent and your using a 4" fan you may not need to dial it back... if you don want to get lower price controller I recommended to SMS single fan one , they are about £60 in the UK (that where I'm, from). I use one and it does the job great but still makes the fan hum a bit. I had to build silencer boxes for my fans to sit in to deaden the noise.
When it come to fan controllers, the more you spend the better the product... with out going into the workings of fan controllers the really pricey one's don't make your fan hum.