Is the top fill cap rotating all the way closed? One issue with the Mini many found was that it was too tight and that lip on the sealing pad would prevent it from sealing completely. Snipping that off did the trick for my Mini (didn't have to on my full size). Anyways, seals and tank pressure are the name of the game for leaking. That of course could be a variety of things. Check all the o-rings making sure there are no tears, this includes the oring at the bottom of the coilhead. Check that top gasket in the top fill portion. Make sure they're all solid, no tears and that they're sealing right. Also a little pro tip is take a drop of juice on each o-ring and rub it around, this will lube it up a bit so it's not gripping too tight onto the glass/metal, this will make it less likely to rip when you dismantle the thing the next time.
If all your connections are sealing right you shouldn't have any leaking issues. However the cotton inside isn't a complete seal, if you leave the tank alone too long it will eventually seep through, this should take days though unless it's an old coil (replace every 15-25ml, or weekish). If you are going to store the tank for longr than a day without use close the airflow that'll add some time, and if you're waking up to it having leaked you might try just closing the airflow every night.
Anyways, my 2cents.
EDIT: I've used 50/50 all the way to max VG in the TFV4 without issues, so it shouldn't be your juice, though in above average hot weather the viscosity changes which can be an issue, all the more reason to never leave your vape in a hot car.
