spend the money and get a good 4" inline fan from Elicient or Fantech or Vortex or someone like that. They will remove more cfm, more consistently, and can pull harder (I swear an expensive 4" inline fan with the same cfm as a cheaper one will out blow it). They will also last longer and generally be much quieter. Grab yourself a speed controller and at 75% power the 4" 300cfm fans are near dead quiet behind a door or wall.