My daughter and her boyfriend got me a CoolFire IV for Christmas. They ordered the CoolFire online, and went to our local vape shop to get a couple of bottles of eliquid to go with the new mod. While they were at the vape shop, they were discussing the CoolFire IV with the clerk, and he advised them to get a lower nicotine level for me with the CoolFire. I think the clerk may have been assuming that I was going to sub-ohm, since that seems to be all the rage right now, and the CoolFire IV can use a .50 coil. Anyway, they got me a couple of bottles of 6 mg eliquid, instead of my usual 12. Playing around with my new toy and going between the 6 mg eliquid they got me and the 12 mg that I had on hand, I found that the 6 mg is doing OK, and have been sticking with that.
At that time, I was using Evod Glass tanks, going between 1.8 and 2.2 ohm Kanger single coils and 50/50 pg/vg and 30 pg/70 vg, depending on which vendor I was buying from. No problems with the CoolFire IV - I just had to remember that it is variable wattage. Last month I treated myself to a Nautilus Mini, since my daughter really loves the one she got for herself. Wow, what a difference in the coils. I usually only get a few days out of the Kanger coils, maybe a week if I am lucky, depending on the eliquid I am using. I got a full 2 weeks out of the 1.8 ohm Nautilus coil that came preinstalled, and I am in my second week of using the 1.6 ohm that came as a spare.
Have fun with your CoolFire IV. If you are going to sub-ohm, definitely watch your nicotine level. Keep an eye on how different pg/vg ratios and nicotine levels work for you with the equipment you have. Don't be afraid to experiment a little, but please do stay within the parameters given in your user manual for safety's sake.