Yield Master II 6" w/600W HPS is what I am running and it is a nice hood. The duct size is one diff and the other main diff is the size of the reflector.
L x W x H
YM II 4"= 20 1/4 x 21 x 7 1/2
YM II 6"= 18 1/2 x 22 1/2 x 7 1/4

This will play into how the light is cast (ie footprint). A larger reflector will cover a larger space with light.(to a point)
31steel Reviewed by 31steel on . 4" reflectors vs 6" and 10" Hey guys, Found a great hanging reflector for my 1000w HPS and MH bulbs. I even have a nice ballast and all the fixins for proper plant growth. As a matter of fact, here's the URL to the exact reflector I had in mind: Sunlight Supply: Yield Master™ II Classic 4 inch Reflector - Reflectors, Hoods, Shields & Parts - Grow Lighting Systems, Fixtures, Movers, Ballasts, Sockets, Reflectors & Components - Horticulture Source Now, the tricky thing is, while I know the reflector here can Rating: 5