6"fan to 4" ducting

Growbox size 6h x3w x 2d = 36cuft.
400w lights with cool tube.

I have 6" 218cfm fan. It`s alot easier to find 4" duct supplies from places like Loewes or Home Depot. Will this box vent good enough if I use all 4" stuff right from fan?

sounds like it should work just fine. you may not be operating at peak performance by venting a 6 through a 4 though. i think its just a bit harder on the fan though.