Ideal temp is 76 - 78 degrees F.

That said, my bloom area has been running approx 83 degrees the last few days due to it being hot and sticky where I live. (I haven't turned on my a/c yet as we're heavily wooded and using attic fan) I'm using differing combos of 150w and 250w HPS lamps.

The ladies don't like the temp geting much above 82 and at 85 or more, the blooming action pretty much stops, which is counterproductive.

My suggestions to you:
1. Upgrade your fan to at least 6" and add additional intakes or increase size of existing intake(s). The 4" inline fan will never move enough air to cool an HPS in that small an area - I don't care what size the lamp is.
2. Make the room cooler if possible. What is the ambient temp of the room where the grow box is located? That plays a huge part in keeping your temps down, especially when using a small HPS without additional bulb cooling (cool tubes, fans and venting, etc).

Hope this helps.
ipe: cb