My grow tent will be a 4x4 with 600w HPS. I'll be using an 8" exhaust fan (let me know if that's good) Wondering if I would need an intake fan? I want this little project to be as stealthy as it can - I live in an apartment.
Yes i always recommend an intake fan, although for that setup it wouldn't be absolutely necessary. As for the 8" fan it matters the CFM it produces more then the inches it is. How much CFM is it rated for? Also if stealth is very important get a carbon filter for the exhaust fan.
I personally wouldn't use an intake fan. The screens on the bottom of the tent should be open and used to bring in fresh air with negative pressure from your exhaust system. The more air being pulled out through your exhaust, the greater the negative pressure and more air being pulled into your tent through the intake screens at the bottom. You should have some kind of air flow inside the tent though, some clip of fans or something that can help move the air around the canopy and ultimately to your carbon filter and exhaust system. If you do use an intake fan you want the intake to move less air, CFMs, then the exhaust system and strategically place it so you maintain negative pressure at the bottom of the tent. If you don't maintain negative pressure the smell of your plants during flowering will be pushed out of the tent into your apartment. Even with a carbon filter and exhaust. Just my two cents.
If your using a quality brand exhaust fan, you shouldn't need an intake fan. An 8" fan as long as the ducting isn't too long should be plenty powerful enough