damn, thats a downer

don't know what to advise in regard the tfv4 as my glass has held up very well, especially considering the extremities it undergoes.
i have had to replace a seal mind, as the first one eventually perished to the point that it wasn't doing its job.
i can offer you some good recommendations though for an alternative, both for the subtank and tfv4. the steam crave aromamiser - it's a rebuildable dual coil setup that's easy to build and very impressive.
there are various versions, mine being an earlier one. it has a couple of negatives mind but nothing too significant as to be off-putting. those are the fill method which is done by unscrewing the tank from the base which can be considered inconvenient in comparison to easy fill alternatives. also it has a kind of neck on the top as oppose to being just flat topped, which can make drip tips look a little less nice than they might.
apart from those slight niggles though it really is a great tank. there are newer revisions too which improve on those issues. i'm just awaiting arrival of the latest revision which has top, fill, is significantly larger, and has flat top.
it's called the aromamiser supreme and is steam craves entry into a new generation of what are essentially, velocity style drippers incorporated into large sized, gravity fed tank type things.
there are others which are very similar in design such as the Augvape Boreas and the upcoming, TF-RDTA by Smok. all of which receive rave reviews.