I removed this fan quite a while back, and the inside of my enclosure would get HOT. To the point where it was actually a little painful to touch the rods. It also increased my warping problems a bit, but I am not sure why.

Last night I got fed up with the stock filament feeding system (I was printing something large and the extruder would choke up near the front of the build plate), and put a box on top for now (and removed the SD4 top). Now, this box is sealed all the way to the top, but it has gaps on the bottom. The enclosure is a bit cooler now, and interestingly, my warping issues seem to have disappeared as well. ... You'd be amazed how much of a stress the stock SD4 filament feed is on the drive system, also. I saw immediate improvement in just about every aspect, from the drive system to extrusion.

That's interesting.  Sans the box, that's kinda the filament orientation I've been using for some time.  I have a spool mounted behind / above the SD frame, with the spool axis parallel to the top back edge of the frame.  A kind of 'guide' thingy that rocks on a base magneted along the back edge frame rail that the filament passes thru, about 4" above the frame edge.  Then the whole top of my case is cut away.  I haven't bothered with trying to re-enclose it in a bigger box, then again I don't have any sort of inside fan like the newer SD4's apparently do, either.

I'll try to cap some pics one of these days and edit this post if allowed, or just post elsewhere.  For me reducing the feed friction was a huge necessity to keep prints from drifting layer to layer.  My longest print has been about 5.5 hours, as long as I don't have the ceiling fan on above the SD I don't get intralayer cracks or much warpage during the print time.  I do keep the machine in typical indoor temps (say 68 - 80F, depending on season).

I've seen at least one user here put a fan on the side of the frame, which sucks air right across the print. I might give that a go once I get some plexi. Maybe make it an additional GCode fan.

I just noticed three of my back fan blades are missing. I probably broke them with a screwdriver at same point.
I wonder if that has anything to do with my warping getting so much worse lately...

Last night I got fed up with the stock filament feeding system (I was printing something large and the extruder would choke up near the front of the build plate), and put a box on top for now (and removed the SD4 top). Now, this box is sealed all the way to the top, but it has gaps on the bottom. The enclosure is a bit cooler now, and interestingly, my warping issues seem to have disappeared as well. I was able to print this without any warping at 100% infill: http://www.thingiverse.com/thing:97734 Now I just need to install it You'd be amazed how much of a stress the stock SD4 filament feed is on the drive system, also. I saw immediate improvement in just about every aspect, from the drive system to extrusion.

The whole point of this post is, I think without that fan it gets too hot in there, partially because of the short distance between the enclosure ceiling and the bed. While you want the enclosure to be heated, I think it is very possible to overdo it and have negative consequences. If the fan is keeping it somewhat cooler in there, I'd leave it alone. Airflow is good.

just few days old dude

SD 4 has the motherboard inside the case but it has its own little enclosure and there's also a fan inside the enclosure cooling the motherboard, so the back fan is definitely not for the motherboard.

Woops - must've misread the timeline.  Sorry.  Thanks for the info; hope someone else has a better answer for you.

SD 4 has the motherboard inside the case but it has its own little enclosure and there's also a fan inside the enclosure cooling the motherboard, so the back fan is definitely not for the motherboard.

BTW how did you print that with no overhangs?
What hotend temp? And do you have a print cooling fan?

I too doubted the usefulness of it, and when delivered, my SD4 even had a 'coffee filter-like' material covering it, which I removed.  (I don't seem to have an ABS odor problem like some users report, I'm not sure the filter would have solved that anyway)  At one point during my tweaking, I disconnected that fan, thinking I wanted to keep my enclosure at a warmer temperature, with no airflow. 

In playing with bridges, overhangs, etc., I decided that if the temperature inside the enclosure is TOO warm, my nozzle fan is just blowing hot air on the filament as it exits, and isn't cooling it nearly as well.  I subsequently re-connected the fan, and noticed improvement in overhangs, etc...especially with PLA.  You can see this on my Christmas ornament in the Print Showoff topic; that ball-ornament with a scene inside is an 18-hour print at .1mm, with NO support material.

I've come to the opinion that the purpose of the enclosure is to mitigate sudden changes in room temperature, and airflow is not necessarily a bad thing.  I would guess that this varies with the room environment the printer's in.