Don't need much clearance for the driver, as it is an 18" diameter radiating surface - the volume of air displaced is easily diffused. With the 8" diameter port, it's not as easy, as pressure is higher, making it more prone to experience turbulence. By all means, you should be fine with the 4" clearance at most listening levels, but with deep bass at spirited levels, when the port is being put to good use, you may hear some weird sounds.
How many layers are you planning on for that baffle?
