Need help with 400w tent.

Ok I'm new to this and have got a tent with a 400w bulb and with a 4" extractor fan, the room temp is around 85 and the tent is also around 85 with no lights on, I know I need another fan but how strong in CFM?

Adding another fan won't cool down your tent. You need an a/c in the room. It is impossible to cool down your tent below the temperature the room it's in, unless you stick an a/c in the tent.

How big is this tent? Mr2Shim is right that you cannot cool the area below the ambient temp of the room without AC but 85-90 with light on is ok. Not ideal but ok. By the time you go to bloom at this point it will be closer to fall and the ambient will be lower anyway. Whats the temp with the light and fan on? 4" is ok to cool a 400W. 6" would be better.

Another fan will just push the hot air around the room.

Maybe cold air intake at night from outside?