I've been using this combo for a few years now with no issue (using stock Kanger coils).  The two Top Tanks that I had were getting a little long in the tooth and my Coolfire IV was looking a little beat, so I purchased a new Coolfire IV and two new Kanger Toptanks.

The issue started when using the new Toptanks with the new Coolfire IV.  I'd load up a new coil, fill her up as I always do, and when I'd screw the tank down onto the battery, I'd get a "short circuit" warning.  If I backed the tank off 1/4 to 1/2 turn, everything would be alright and the resistance would read right around where it should (.55-.6 ohms on .5 ohm coils).  When I'd plug the new tanks into my old Coolfire IV there was no issue, I could screw down the tank all the way and everything would read like normal.

Now the issue is starting to happen with my old Coolfire IV.  One tank is randomly doing it (I can screw it all the way down and it'll be fine, but it will occasionally give me a short circuit warning or the ohms will dive down to something like .1-.2) and the other tank is doing it constantly.  Same thing with the new Coolfire, if I back the tank off 1/4-1/2 turn, the issue goes away.

Now, I would try and use one of my old Toptanks to try and rule out or confirm the new tanks as the problem, but the glass is broken on the one I have left so I haven't tried that yet.  The stock coils are from the same batch of coils I've been using for a while, so I don't think they're the issue.  I was thinking it could be the new Coolfire IV, but now that the issue is presenting itself on my old Coolfire, I'm thinking it's not the issue.

Has anyone experienced anything like this?  I don't want to just start throwing money away on new tanks (considering I have a decent stock of replacement coils for the Kanger tanks).

I had that happen to me on my Coolfire IV with an Aspire Cleito tank.  Never happened on my other mods though.  The little girl at the local shop said she's seen it quite a bit with the Coolfires.  

Never saw it with my OBS Engine tank on the Coolfire though.