cfm ,diameter fan size question

for a room i will be running , i will need a exhaust fan that moves 864 cfms but i am also using it to cool my light but my adapter for the light is 4 inches so do i get a step ring to a bigger size such as a 8 inch or 10 inch or just use the same diameter as my light? i am trying to exhaust my air out the room via a carbon filter then thru my light and out of the room

For as much air as you need to move I would recommend a second fan. The 4" outlet from the light will restrict the airflow, as will any length of ducting. I've read varying numbers, but pulling the air through ducting will reduce the effective CFM rating of any fan. If you need to move 864 CFM, you're already calling for a large fan.

The good news is that 2 fans to split the work will end up costing little more than one huge fan. Hook one up dedicated to the light (I'd step it up to 6"), and have one for exhaust (6" or 8")