I have only seen the 4" myself. The flow rate will not suffer too much depending on the length of the section from 6" to 4". The ridges and bends kill the flow. PVC? Then split and combine the vent tubing perhaps. Doesn't sound real practical though.
A-a-h-h, The Orang beat me to it! Air conditioning ducts (flexible) are available, in 2" increments, up to 14".
'The reason i need this stuff i've got a 600W HPS on the way with a cool tube and a 150CFM "Can Fan".' Couldn't you get the conduit from the same place you got the light? BB, you have a point there; 'old Peter Built exhaust stacks' are connected to the old peterbilt engines with 6" flex pipe. Although it would be very heavy and might not have enough flex to go where M4 wants it to go:>)
Hey M4N! Glad to see you back in the "Hoked-up World"! I look at it this way, if there's a problem; there has to be a solution!