Does it get cold where you are?
Is the shed insulated?
if yes and no, then youre going to have problems with cold when your light is out during flowering.

About the venting, I would seal the windows and vent through a hole in the wall....much more controlled that way.
And yes, a 1000 Watt would be better, but the issue is heat....you will need some very good ventilation with a 1000 W in 16 Sq. Ft.
600 will be fine.
And I would get a HPS. If you have the money, get a switchable ballast. Or, like hombre above me said, get the HPS and use blue spec CFL's around it.

Good luck and keep us posted!
hanayama Reviewed by hanayama on . Appreciate HELP with my setup please! I have a 4x4 area in a shed outside. I am going to get a 600w hps light. Does mh really make a difference for vegetative if I'm trying to keep costs down or will the one do fine with both stages? Also, there are 3 windows. Should I completely seal them? I'm trying to figure out the best thing to do for ventilation and co2 issues without purchasing a lot of extra equipment IF I don't have to. Won't be a problem to cut some vents if necessary. Also, would a 1000w be even better for this area or Rating: 5