my 4ft x 2ft x 6ft tent needs a real fan?

first time buying a real fan for my tent. ive been using a 8in booster fan. its been working fine cause the tent got windows at the top. i was thinking of a hydro farm 6in but my hydro girl said i could use a 4in without a speed control. any input? but it gets hot in the summer so i was still thinking of a 6in. also i would be hanging it so i was wondering about noise? one more thing would i be able to use the same fan to vent out my 2ft x 2ft veg room at same time? or get a 8in to do it? thanks. oh yeh 400 watt..

The size of the fan doesn't really matter that much, it's how much air it moves, and how much noise it makes. Bigger fans have an easier time moving more air more quietly.
I wouldn't use the same fan to vent your veg room and your tent. It's more complicated to do the venting that way and a 2'x2' area doesn't need much airflow to vent properly. You could get a little 4" 80cfm online fan at Home Depot for $25. It's a really quiet little fan.
Your tent is only 48 ft^3, so you could possibly use the little 4" for that as well. You want to replace the air in your tent in a minute. If you're doing the light and the room on the same fan you might want something a little bigger because 80cfm is not enough to cool the light well.

thanks man